Method and apparatus for surgery of the cornea using short laser pulses having shallow ablation depth

ABSTRACT

A laser-based method and apparatus for corneal surgery. The present invention is intended to be applied primarily to ablate organic materials, and human cornea in particular. The invention uses a laser source which has the characteristics of providing a shallow ablation depth (0.2 microns or less per laser pulse), and a low ablation energy density threshold (less than or equal to about 10 mJ/cm 2 ), to achieve optically smooth ablated corneal surfaces. The preferred laser includes a laser emitting approximately 100-50,000 laser pulses per second, with a wavelength of about 198-300 nm and a pulse duration of about 1-5,000 picoseconds. Each laser pulse is directed by a highly controllable laser scanning system. Described is a method of distributing laser pulses and the energy deposited on a target surface such that surface roughness is controlled within a specific range. Included is a laser beam intensity monitor and a beam intensity adjustment means, such that constant energy level is maintained throughout an operation. Eye movement during an operation is corrected for by a corresponding compensation in the location of the surgical beam. Beam operation is terminated if the laser parameters or the eye positioning is outside of a predetermined tolerable range. The surgical system can be used to perform surgical procedures including removal of corneal scar, making incisions, cornea transplants, and to correct myopia, hyperopia, astigmatism, and other corneal surface profile defects.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 07/740,004, filed Aug. 2, 1991, entitled “TwoDimensional Scanner-Amplifier Laser”.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to methods of, and apparatus for, surgeryof the cornea, and more particularly to a laser-based method andapparatus for corneal surgery.

[0004] 2. Related Art

[0005] Art Related to the Inventive Method and Apparatus for Surgery

[0006] The concept of correcting refractive errors by changing thecurvature of the eye was brought forth early on, as illustrated in thenotable mechanical methods pioneered by J. Barraquer. These mechanicalprocedures involve removal of a thin layer of tissue from the cornea bya micro-keratome, freezing the tissue at the temperature of liquidnitrogen, and re-shaping the tissue in a specially designed lathe. Thethin layer of tissue is then re-attached to the eye by suture. Thedrawback of these methods is the lack of reproducibility and hence apoor predictability of surgical results.

[0007] With the advent of lasers, various methods for the correction ofrefractive errors have been attempted, making use of the coherentradiation properties of lasers, and the precision of the laser-tissueinteraction. A CO₂ laser was one of the first to be applied in thisfield. Peyman, et al., in Ophthalmic Surgery, vol. 11, pp. 325-9, 1980,reported laser burns of various intensity, location, and pattern wereproduced on rabbit corneas. Recently, Horn, et al., in the Journal ofCataract Refractive Surgery, vol. 16, pp. 611-6, 1990, reported that acurvature change in rabbit corneas had been achieved with a Co:MgF₂laser by applying specific treatment patterns and laser parameters. Theability to produce burns on the cornea by either a CO₂ laser or aCo:MgF₂ laser relies on the absorption in the tissue of the thermalenergy emitted by the laser. Histologic studies of the tissue adjacentto burn sites caused by a CO₂ laser reveal extensive damagecharacterized by a denaturalized zone of 5-10 microns deep anddisorganized tissue region extending over 50 microns deep. Such lasersare thus ill-suited to corneal laser surgery.

[0008] In U.S. Pat. No. 4,784,135, Blum et al. discloses the use offar-ultraviolet radiation of wavelengths less than 200 nm to selectivelyremove biological materials. The removal process is claimed to be byphotoetching without requiring heat as the etching mechanism. Medicaland dental applications for the removal of damaged or unhealthy tissuefrom bone, removal of skin lesions, and the treatment of decayed teethare cited. No specific use for cornea surgery is suggested, and theindicated etch depth of 150 microns is too great for most cornealsurgery purposes. Further, even though it is suggested in this referencethat the minimum energy threshold for ablation of tissue is 10 mJ/cm²,clinical studies have indicated that the minimum ablation threshold forexcimer lasers at 193 nm for cornea tissue is about 50 mJ/cm².

[0009] In U.S. Pat. No. 4,718,418, L'Esperance, Jr. discloses the use ofa scanning laser characterized by ultraviolet radiation to achievecontrolled ablative photode-composition of one or more selected regionsof a cornea. According to the disclosure, the laser beam from an excimerlaser is reduced in its cross-sectional area, through a combination ofoptical elements, to a 0.5 mm by 0.5 mm rounded-square beam spot that isscanned over a target by deflectable mirrors. (L'Esperance has furtherdisclosed in European Patent Application No. 151869 that the means ofcontrolling the beam location are through a device with a magnetic fieldto diffract the light beam. It is not clear however, how the wave frontof the surgical beam can be affected by an applied magnetic to anypractical extent as to achieve beam scanning.) To ablate a cornealtissue surface with such an arrangement, each laser pulse would etch outa square patch of tissue. Each such square patch must be placedprecisely right next to the next patch; otherwise, any slightdisplacement of any of the etched squares would result in grooves orpits in the tissue at the locations where the squares overlap and causeexcessive erosion, and ridges or bumps of unetched tissue at thelocations in the tissue where the squares where not contiguous. Theresulting minimum surface roughness therefore will be about 2 times theetch depth per pulse. A larger etch depth of 14 microns per pulse istaught for the illustrated embodiment. This larger etch depth would beexpected to result in an increase of the surface roughness.

[0010] Because of these limitations of laser corneal surgery systems, itis not surprising that current commercial manufactures of excimer lasersurgical systems have adopted a different approach to corneal surgery.In U.S. Pat. No. 4,732,148, L'Esperance, Jr. discloses a method ofablating cornea tissue with an excimer laser beam by changing the sizeof the area on the cornea exposed by the beam using a series of masksinserted in the beam path. The emitted laser beam cross-sectional arearemains unchanged and the beam is stationary. The irradiated flux andthe exposure time determines the amount of tissue removed.

[0011] A problem with this approach is that surface roughness willresult from any local imperfection in the intensity distribution acrossthe entire laser beam cross-section.

[0012] Furthermore, the intended curvature correction of the cornea willdeviate with the fluctuation of the laser beam energy from pulse topulse throughout the entire surgical procedure. This approach is alsolimited to inducing symmetric changes in the curvature of the cornea,due to the radially symmetrical nature of the masks. For asymmetricrefractive errors, such as those commonly resulting from corneatransplants, one set of specially designed masks would have to be madefor each circumstance.

[0013] Variations of the above technique of cornea ablation have alsobeen developed for excimer lasers. In U.S. Pat. No. 4,941,093, Marshallet al. discloses the use of a motorized iris in a laser beam path tocontrol the beam exposure area on the cornea. In U.S. Pat. No.4,856,513, Muller discloses that re-profiling of a cornea surface can beachieved with an erodible mask, which provides a pre-defined profile ofresistance to erosion by laser radiation. This method assumes a fixedetch rate for the tissue to be ablated and for the material of theerodible mask. However, etch characteristics vary significantly,depending on the type of the materials and the local laser energydensity. The requirements of uniformity of laser intensity across thebeam profile and pulse to pulse intensity stability, as well aslimitation of the technique to correct symmetric errors, also apply tothe erodible mask method.

[0014] Another technique for tissue ablation of the cornea is disclosedin U.S. Pat. No. 4,907,586 to Bille et al. By focusing a laser beam intoa small volume of about 25-30 microns in diameter, the peak beamintensity at the laser focal point could reach about 10¹² watts per cm².At such a peak power level, tissue molecules are “pulled” apart underthe strong electric field of the laser light, which causes dielectricbreakdown of the material. The conditions of dielectric breakdown andits applications in ophthalmic surgery had been described in the book“YAG Laser Ophthalmic Microsurgery” by Trokel. Transmissive wavelengthsnear 1.06 microns and the frequency-doubled laser wavelength near 530 nmare typically used for the described method. The typical laser mediumfor such system can be either YAG (yttrium aluminum garnet) or YLF(yttrium lithium fluoride). Bille et al. further discloses that thepreferred method of removing tissue is to move the focused point of thesurgical beam across the tissue. While this approach could be useful inmaking tracks of vaporized tissue, the method is not optimal for corneasurface ablation. Near the threshold of the dielectric breakdown, thelaser beam energy absorption characteristics of the tissue changes fromhighly transparent to strongly absorbent. The reaction is very violent,and the effects are widely variable. The amount of tissue removed is ahighly non-linear function of the incident beam power. Hence, the tissueremoval rate is difficult to control. Additionally, accidental exposureof the endothelium by the laser beam is a constant concern. Mostimportantly, with the variation in the ablated cross-sectional area andthe etch depth, sweeping the laser beam across the cornea surface willmost likely result in groove and ridge formation rather than anoptically smooth ablated area.

[0015] Other problems that occur with some of the prior art systemsresult from the use of toxic gases as the lasing material. This isparticularly a problem with excimer lasers, which are frequently used inhealth clinic and hospital environments.

[0016] An important issue that is largely overlooked in all theabove-cited references is the fact that the cornea is a living organism.Like most other organisms, corneal tissue reacts to trauma, whether itis inflicted by a knife or a laser beam. Clinical results have showedthat a certain degree of haziness develops in most corneas after laserrefractive surgery with the systems taught in the prior art. Theprincipal cause of such haziness is believed to be surface roughnessresulting from grooves and ridges formed while laser etching.Additionally, clinical studies have indicated that the extent of thehaze also depends in part on the depth of the tissue damage, which ischaracterized by an outer denatured layer beneath which is a moreextended region of disorganized tissue fibers. Another drawback due to arough corneal surface is related to the healing process after thesurgery: clinical studies have confirmed that the degree of hazedeveloped in the cornea correlates with the roughness at the stromalsurface.

[0017] For reliable ablation results, a current commercial excimer lasercorneal surgery system operates at about 150-200 mJ/cm². The etch depthat 193 nm is about 0.5 microns per pulse, and the damage layer is about0.3 microns deep. Light scattering from such a surface is expected.

[0018] It is therefore desirable to have a method and apparatus forperforming corneal surgery that overcomes the limitations of the priorart. In particular, it is desirable to provide an improved method ofcornea surgery which has accurate control of tissue removal, flexibilityof ablating tissue at any desired location with predetermined ablationdepth, an optically smooth, finished surface after the surgery, and agentler surgical beam for laser ablation action.

[0019] The present invention provides such a method and apparatus. Theinvention resolves the shortcomings of the current corneal surgicalsystems, including the use of toxic gases, limitations stemming fromcorrecting only symmetric errors in the case of excimer laser systems,the extensive damage caused by Co:MgF₂ and CO₂ laser systems, and theuncertainty of the etch depth in the case of YAG or YLF laser systems.

[0020] Art Related to the Scanner-Amplifier Laser Invention

[0021] The control of laser beam positioning has become a key element inmany fields of applications, such as image processing, graphic display,materials processing, and surgical applications involving precisiontissue removal.

[0022] A general overview of the topic is given in “A Survey of LaserBeam Deflection Techniques”, by Fowler and Schlafer, Proceedings ofIEEE, vol. 54, no. 10, pages 1437-1444, 1966.

[0023] U.S. Pat. No. 3,432,771 to Hardy et al. issued Mar. 11, 1969,disclosed an apparatus for changing the direction of a light beam in anoptical cavity. The cavity consists of a focussing objective locatedbetween two reflectors, such as curved mirrors. The relative position ofone center of curvature with respect to the other center of curvaturecan be controlled by positioning of one of the mirrors. Points on thereflectors are located at the object and the image positions for theobjective. When the active medium is suitably excited, the orientationof the lasing mode, and hence the position of the spots of light, isdetermined by the effective angular positions of the reflectors.

[0024] U.S. Pat. No. 3,480,875 to Pole, issued Nov. 25, 1969, discloseda laser cavity which was set up between a pair of plane mirrors. Atleast one active laser element is located between the mirrors. A pair oflens systems are positioned between the mirrors so that they have acommon focal plane between them. A Kerr cell, polarizers, and acompensator suppress light oscillation along certain reflector pathswithin the cavity, thereby setting up preferred modes of oscillationalong other paths. Laser emission occurs along the preferred paths.

[0025] U.S. Pat. No. 3,597,695 to James E. Swain, issued Aug. 3, 1971,disclosed an apparatus for amplifying laser light by multiple passesthrough a lasing material in a single laser cavity. A single amplifierstage achieved what had been accomplished by several stages. This isaccomplished by a switching mechanism which directs a laser beam intoand out of the cavity at selected time intervals, thereby enablingamplification of low intensity laser pulses to an energy level near thedamage limits of the optical components of the system.

[0026] U.S. Pat. No. 4,191,928 to John L Emmett, issued Mar. 4, 1980,disclosed a high energy laser system using a regenerative amplifierwhich relaxes all constraints on laser components other than theintrinsic damage level of matter, so as to enable use of available lasersystem components. This can be accomplished by use of a segmentedcomponent spatial filter.

[0027] Many techniques have been developed for controlling the laserbeam direction. For the purpose of this invention, this discussion willbe limited to the speed, accuracy, and the scan angle range of differentdevices used in a random access mode.

[0028] Galvanometer mirror scanners have a large scan angle range.However, the mechanical response due to the balance of the coil and theapplied magnetic field is limited to a few hundred hertz. The settlingtime and oscillation about the equilibrium point further limits theaccuracy attainable with such devices.

[0029] Mirrors positionable with piezo actuators are capable of anaccurate hunt-free movement response of up to tens of kilohertz,depending on the design of the mounts. The typical scan angle is on theorder of a few milli-radians. Methods to enhance the scan angle havebeen proposed by J. Schlafer and V. J. Fowler, “A Precision, High Speed,Optical Beam Scanner”, Proceedings, International Electron DevicesMeeting, 1965. In their report, multiple scanning piezo-mirrors whereused to intercept a laser beam, such that the scan angle of each scannercontributes to the total effect, which is the sum of all scan angles.This device requires many individual scanner units, which multiplies ineconomic cost with the number of units. The mirror size also limits thenumber of units that can be used before the beam will miss the lastmirror.

[0030] Furthermore, both of the above methods are applicable in onedimensional scanning only. For two-dimensional scans, an additionalunit, which is either an identical or a mix with another device, must beprovided for scanning in the other dimension, doubling cost and spacerequirements.

[0031] In U.S. Pat. No. 3,480,875 to R. V. Pole, disclosed is a scanninglaser device, in which the spatial orientation of the laser beam in theresonant cavity is controlled by passing through a combination of aretardation plate and a Kerr cell inside the laser cavity. At a specificangle, as determined by the Kerr cell, loss is minimum for the laserbeam, and therefore the laser beam will oscillate in that preferreddirection. While this method allows scanning of large angles, the scanspeed is limited by the laser build-up time, for which the laser beamintensity will be re-established at each new beam direction. Anotherdrawback of this arrangement is the variation in the laser intensityduring the laser build-up.

[0032] In U.S. Pat. No. 3,432,771 to W. A. Hardy, disclosed is anotherscanning laser, in which the optical cavity consists of a focussingobjective, and spherical reflectors, or equivalent optics which consistof a lens and a plane mirror. The scan angle is magnified mosteffectively in an optical arrangement in which the two end reflectorsform a nearly concentric cavity with the focussing lens at the center offocus. The drawback is that the cavity tolerates diverging beams tobuild up inside the cavity, as illustrated in FIG. 1 of the patent,hence the laser output has a high content of multiple transverse modes.By increasing the radius of curvature of the scan mirror and keeping itslocation fixed, the multi-mode content can be reduced, but the scanrange will approach that of the actual scan angle with a possible smallmagnification factor. As suggested by its preferred embodiment with anelectro-optical beam deflector, the scan angle will be only a fewmilli-radians if a near diffraction-limited laser beam is to beproduced.

[0033] It would thus be desirable to have a scanner-amplifier unit whichaccepts a low energy laser pulse and emits an amplified laser pulse at apredetermined angular positions in two dimensions. The present inventionprovides such a unit.

SUMMARY OF THE INVENTION

[0034] The optimal surgical method for the cornea can be bestappreciated from the characteristics required of the cornea to performits important functions. The corneal surface is the first opticalinterface where all light enters into the eye and thereafter formsimages at the retina. Corneal shape, degree of smoothness, and clarityall determine visual acuity and the contrast sensitivity of the visionsystem. Hence, the importance of the optical quality of the corneacannot be over-emphasized.

[0035] The physical limits on the allowable surface roughness of thecornea can be understood by noting the following facts: humanphoto-sensors on the retina have a wavelength sensitivity range of about380-850 nm in the optical spectrum; surface roughness exceeding half ofthe wavelength within the sensitivity range will act as light scatteringcenters; therefore, any inhomogeneity of the cornea surface or theinside stromal layer ideally should be kept at or below 0.2 microns toachieve an optically-smooth corneal surface.

[0036] The present invention recognizes that an optically smooth cornealsurface and a clear cornea (including post-operative clarity) are allcritical to successful refractive corneal surgery. The invention wasdeveloped with a particular view to preserving these characteristics.

[0037] The preferred method of performing a surface ablation of corneatissue or other organic materials uses a laser source which has thecharacteristics of providing a shallow ablation depth (0.2 microns orless per laser pulse, and preferably 0.05 microns or less per laserpulse), and a low ablation energy density threshold (less than or equalto about 10 mJ/cm²), to achieve optically smooth ablated cornealsurfaces. The preferred laser system includes a Ti-doped Al₂O₃ laseremitting from about 100 up to about 50,000 laser pulses per second, andpreferably about 10,000 laser pulses per second. The laser wavelengthrange is about 198-300 nm, with a preferred wavelength range of about198-215 nm, and a pulse duration of about 1-5,000 picoseconds. The laserbeam cross-sectional area varies from 1 mm in diameter to any tolerablyachievable smaller dimension, as required by the particular type-ofsurgery.

[0038] According to the present invention, each laser pulse is directedto its intended location on the surface to be ablated through a laserbeam control means, such as the type described in a co-pending,commonly-owned patent application for an invention entitled “TwoDimensional Scanner-Amplifier Laser” (U.S. patent application Ser. No.07/740,004). The present invention also discloses a method ofdistributing laser pulses and the energy deposited on a target surfacesuch that surface roughness is controlled within a specific range.

[0039] Additionally, the preferred apparatus for performing cornealsurgery includes a laser beam intensity monitor and a beam intensityadjustment means, such that constant energy level is maintainedthroughout the operation. The location for the deposition of each pulseof laser energy relative to the surface to be ablated is controlled bymonitor means such that eye movement during the operation is correctedfor by a corresponding compensation in the location of the surgicalbeam. Provision for a safe and efficacious operation is included in thepreferred apparatus, such that the operation will be terminated if thelaser parameters or the eye positioning is outside of a predeterminedtolerable range.

[0040] According to the present invention, various surgical procedurescan be performed to correct refractive errors or to treat eye diseases.The surgical beam can be directed to remove cornea tissue in apredetermined amount and at a predetermined location such that thecumulative effect is to remove defective or non-defective tissue, or tochange the curvature of the cornea to achieve improved visual acuity.Incisions on the cornea can be make in any predetermined length anddepth, and they can be in straight line or curved patterns.Alternatively, circumcisions of tissue can be made to remove an extendedarea, as in a cornea transplant.

[0041] Although the primary use of the present invention is inophthalmology, the laser ablation process can be applied in areas ofneurology for microsurgery of nerve fibers, cardiology for the removalof plaque, and urology for the removal of kidney stones, just to mentiona few possible uses. The present invention can also be useful forapplications in micro-electronics in the areas of circuit repair, maskfabrication and repair, and direct writing of circuits.

[0042] The present invention provides an improved method of corneasurgery which has accurate control of tissue removal, flexibility ofablating tissue at any desired location with predetermined ablationdepth, an optically smooth finished surface after the surgery, and agentle surgical beam for laser ablation action.

[0043] The present invention also discloses a new method of reshaping acornea surface with an optically smooth finish by depositing the laserenergy in a prescribed pattern at predetermined locations. This isaccomplished with high speed, precision control of the beam location, asdisclosed in co-pending U.S. application Ser. No. 07/740,004 for aninvention entitled “A Two Dimensional Scan-Amplifier Laser.”

[0044] The present invention also discloses a means to improve accuracyand reproducibility of eye surgery by adjusting the surgical beamdirection to compensate for any eye movement during the surgicalprocedure. In addition, the surgical beam intensity, beam intensityprofile, diameter, and location are monitored and maintained during thesurgery.

[0045] Objects with respect to the Inventive Method and Apparatus forSurgery

[0046] In accordance with the above discussion, these and otherfunctions can be accomplished according to the teachings of the presentinvention, which provides a new and improved laser source, providing agentler surgical beam and a shallower tissue etch depth than taught inthe prior art.

[0047] It is another object of the present invention to provide animproved apparatus and method for removing organic materials from thesurface of living or non-living objects. The present invention isspecifically useful for the ablation of tissue on the cornea.

[0048] It is another object of the present invention to provide a methodof ablating cornea or other organic materials to achieve an opticallysmooth surface.

[0049] It is another object of the present invention to provide newmeans of laser cornea surgery, with a new laser source emitting a largenumber of laser pulses (about 100 to 50,000 laser emissions per second),each of which etches a shallow depth (about 0.2 microns or less) of thecornea tissue.

[0050] It is another object of the present invention to provide newmeans of laser cornea surgery, with a new laser source emitting awavelength of about 198-300 nm, with a preferred range of about 198-215nm, and a pulse duration of about 1-5,000 picoseconds.

[0051] It is another object of the present invention to provide means ofdepositing surgical laser beam energy with a beam control as describedin co-pending U.S. application Ser. No. 07/740,004, for an inventionentitled “A Two Dimensional Scan-Amplifier Laser,” to achieve exactpositioning of each laser pulse.

[0052] It is another object of the present invention to provide agentler ablative surgery, with significantly reduced damage and traumaof the tissue or organic materials adjacent to the ablation site, incomparison to the prior art.

[0053] It is another object of the present invention to provide means toremove cornea tissue or other organic materials at predeterminedlocations, over predetermined areas, and with predetermined depths ofablation.

[0054] It is a specific object of the present invention to correctrefractive errors, including myopia, hyperopia, and astigmatism, of theeye. It is another specific object of the present invention to correctrefractive errors that may be spherically symmetric or asymmetric.

[0055] It is another object of the present invention to remove scars,tumors, and infected or opaque tissue on the cornea.

[0056] It is another object of the present invention to provide animproved method for performing a cornea transplant operation. It isanother object of the present invention to provide an improved method ofmaking incisions on the cornea, to achieve correction of myopia and/orastigmatism.

[0057] It is another specific object of the present invention that theinventive methods be automated with computer control for accurate andsafe operation.

[0058] It is yet another specific object of the present invention toprovide control means for compensating for eye movement during anoperation by making a corresponding adjustment of the surgical beamlocation.

[0059] Objects with respect to the Scanner-Amplifier Laser Invention

[0060] The following objects are in accordance with the teachings of theco-pending, commonly-owned patent application for the invention entitled“Two Dimensional Scanner-Amplifier Laser” (U.S. patent application Ser.No. 07/740,004).

[0061] An object in accordance with the present invention is to providea scanner-amplifier unit which accepts a low energy laser pulse andemits an amplified laser pulse at a predetermined angular positions intwo dimensions.

[0062] It is another object of this invention to disclose a constructionof a high speed scanner-laser amplifier system, which has the capabilityof large scan angles, and the capability of emitting high quality, neardiffraction limited laser beam. The scanner of the present invention canposition a laser beam in two dimensions in a random access mode at highspeed.

[0063] It is another object of the invention that the scanner-amplifiersystem generate ultra-short laser pulses of 1-500 picoseconds durationat a multi-kilohertz repetition rate, and that the energy of each laserpulses is amplified in a controlled manner to a desired level up to thedamage level of the optical components.

[0064] It is another object of the invention that the laser medium is tobe pumped by a plurality of laser beams in a longitudinal direction,such that high excitation density is achieved in the laser medium.

[0065] It is another object of the invention that the scanner-amplifiersystem can place an individual high energy laser pulse at a preciselyintended angular location in a two-dimensional space.

[0066] It is yet another object of this invention to construct aTi:Al₂O₃ laser with a high laser pulse rate, in the range of 1000 to50,000 pulses per second, and with high average laser power, in therange of several watts or higher.

[0067] It is an object of this invention that each laser pulse has highpeak power, and a short pulse duration, of sub-picoseconds to hundredsof picoseconds.

[0068] Still another object of this invention is to generate stable andhigh conversion efficiency in the second harmonic laser wavelength,which is used to generate population inversion in the Ti:Al₂O₃ lasermedium.

[0069] It is an object of this invention to provide a novel method toattain high pump power in the second harmonic wavelength for theTi:Al₂O₃ laser.

[0070] It is an object of this invention to propose a novel method toattain high pump power in an end-pumping configuration for the Ti:Al₂O₃laser.

[0071] The preferred method for controlling the direction of the laserbeam consists of a pair of scanning mirrors driven by piezo actuators.The mirror pair are driven in tandem. The scan angles of the mirror pairare summed and amplified by an optical arrangement. Two convergentspherical lenses of un-equal focal length are arranged between thescanning mirrors in such a way that a laser beam will be travellinginside the cavity in which the boundary is defined by the scan mirrors.For each round trip of the laser beam inside the cavity, the angle ofthe laser beam to an exit window increases as a multiple of the actualscan angles of the scan mirrors.

[0072] In accordance with this invention, the direction of the laserbeam emitted from the scanner-amplifier system is controllable in twodimensions, at high speed, and with high precision.

[0073] In a preferred embodiment, the laser beam is generated by anamplifying means with a seeding laser pulses. Optical retardation plate,Pockels cell, and polarization dependent optical elements are used forthe control of a seed laser beam and for directing that laser beam inthe amplifier cavity. A laser gain medium is included in the cavity.Means for exciting the laser medium, and for generating multi-kilohertz,ultra-short duration laser pulses, are disclosed in the invention. Meansfor controlling the timing and the synchronization of the seed pulse,the pump source, and the amplified laser pulses inside thescanner-amplifier cavity are also provided.

[0074] It is an object of this embodiment to provide a means and methodfor combining a plurality of laser beams to provide a high power laserbeam source.

[0075] It is another object of the invention to provide a combiner forcombining a plurality of laser beams that does not require any form ofspecific polarization in any of the component beams. It is an object ofsuch a combiner that it can form a beam bundle consisting of largenumber of beams in a small cross section.

[0076] It is yet another object of this invention to provide a novelmethod of combining a plurality of laser beams to provide a high powerlaser beam source for an end-pumping configuration of a laser beam. Thecombiner eliminates limitations imposed by the physical size of the beamsteering optics and the optical mounts (an earlier method of beamcombining relies on the direction of the linear polarization, and thismethod is limited to combining two beams only).

[0077] The details of the preferred embodiments of the present inventionare set forth in the accompanying drawings and the description below.Once the details of the invention are known, numerous additionalinnovations and changes will become obvious to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0078]FIG. 1 is a block diagram of the preferred embodiment of theinventive apparatus.

[0079]FIG. 2A is a side view of a beam diameter sensor, including animaging device, in accordance with the present invention.

[0080]FIG. 2B is a front view of the imaging device of FIG. 2A.

[0081]FIG. 3A is a side view of a beam location sensor, including aphoto-detector, in accordance with the present invention.

[0082]FIG. 3B is a front view of the photo-detector of FIG. 3A.

[0083]FIG. 4A is a perspective view of a vacuum ring used in conjunctionwith the present invention.

[0084]FIG. 4B is a top plan view of the vacuum ring of FIG. 4A.

[0085]FIG. 4C is a block diagram of an eye tracking system in accordancewith the present invention.

[0086]FIG. 5A is a block diagram of a first embodiment of a wavelengthconverter in accordance with the present invention.

[0087]FIG. 5B is a block diagram of a second embodiment of a wavelengthconverter in accordance with the present invention.

[0088]FIG. 6A is a diagram of a first laser beam intensity profile inaccordance with the prior art.

[0089]FIG. 6B is a diagram of a corneal etch profile resulting from thelaser beam intensity profile shown in FIG. 6A.

[0090]FIG. 6C is a diagram of second and third laser beam intensityprofiles in accordance with the prior art.

[0091]FIG. 6D is a diagram of the corneal etch profiles resulting fromthe laser beam intensity profiles shown in FIG. 6C.

[0092]FIG. 6E is a diagram of a pattern of laser beam intensity profilesin accordance with the prior art

[0093]FIG. 6F is a diagram of the laser beam intensity profile resultingfrom the pattern shown in FIG. 6E.

[0094]FIG. 6G is a diagram of a corneal etch profile resulting from thelaser beam intensity profile shown in FIG. 6F.

[0095]FIG. 7A is a diagram of a first level pattern used in scanning atarget with the present invention.

[0096]FIG. 7B is a diagram of a second level pattern used in scanning atarget with the present invention.

[0097]FIG. 7C is a top and side view of a pattern of concentric circlesetched on a cornea using the etch deposition patterns of the presentinvention.

[0098]FIG. 7D is a set of graphs showing the crest-to-trough distancesof level one, level two, and level three etch patterns in accordancewith the present invention.

[0099]FIG. 7E is a diagram showing the measurement axes used to computethe level two and level three crest-to-trough distances of FIG. 7D.

[0100]FIG. 8 is a block diagram of a guide beam unit in accordance withthe present invention.

[0101]FIG. 9A is a top view of a cornea, showing the use of the presentinvention to make radial incisions on the cornea.

[0102]FIG. 9B is a cross-sectional side view of a cornea, showingvariable-depth incisions made using the present invention.

[0103]FIG. 9C is a top view of a cornea, showing the use of the presentinvention to make transverse-cut incisions on the cornea.

[0104]FIG. 10A is a cross-sectional side view of a cornea, showing theuse of the present invention to remove tissue to a desired depth d overa predetermined area on the cornea, and showing an alternative methodfor performing a cornea transplant.

[0105]FIG. 10B is a cross-sectional side view of a cornea, showing theuse of the present invention to correct myopia.

[0106]FIG. 10C is a cross-sectional side view of a cornea, showing theuse of the present invention to correct hyperopia.

[0107]FIG. 11 is a schematic diagram of the integrated scanner-amplifierunit, consisting of a series of intra-cavity optical elements.

[0108]FIG. 12 is a schematic diagram showing a second embodiment of theintegrated scanner-amplifier unit of the invention.

[0109]FIG. 13 is a schematic diagram showing the process of angularamplification for the laser beam inside the scanner-amplifier cavity.

[0110]FIG. 14A is a schematic showing a means of generating stablesecond harmonic laser power.

[0111]FIG. 14B is a perspective showing of a spatial combiner forcombining the plurality of pump beams of FIG. 14A.

[0112]FIG. 14C schematically depicts the combining of the beams of FIG.14A into a single second harmonic beam from the generated beam of FIG.14A.

[0113]FIG. 15A is an exploded perspective view showing a method ofmounting the laser medium.

[0114]FIG. 15B is a cutaway perspective view of the laser medium of FIG.15A enclosed in a water jacket for cooling.

[0115]FIG. 16 is a block diagram showing the electrical connectionsbetween the mode-locked laser driver, the timer-divider circuit, thePockels cell driver, and the Q-switch driver of the pump laser.

[0116]FIG. 17 is diagram showing the synchronizaton between the modelocked laser pulses, the selected laser pulses after the timer-dividercircuit, the Q-switched laser pulses for pumping the gain medium, andthe half-wave optical switch wave form.

[0117] Like reference numbers and designations in the various drawingsrefer to like elements.

DETAILED DESCRIPTION OF THE INVENTION

[0118] Throughout this description, the preferred embodiment andexamples shown should be considered as exemplars, rather thanlimitations on the method and apparatus of the present invention.

[0119] Background Information

[0120] The laser apparatus and system disclosed in this invention is forachieving two principal objectives:

[0121] (1) The damage zone underneath the material ablated by thepresent laser system must be substantially reduced in comparison toprior art laser systems.

[0122] (2) For each laser pulse deposited on the cornea, a definitepredetermined depth of tissue is to be ablated. The ablated depth perlaser pulse must be controllable and about 0.2 microns or less, andpreferably about 0.05 microns or less.

[0123] A brief discussion on the mechanism of the ablation process isuseful to understand how the stated objectives can be achieved by theteaching of the present invention. It is a well-known fact that laserablation can occur when the laser beam intensity is increased beyond acertain level. The actual ablation conditions however, vary depending onthe characteristics of a wide range of laser parameters and thecomposition of the material to be ablated. For the purposes of thepresent invention, only those aspects that are relevant to the twoprincipal objectives will be discussed.

[0124] When laser energy is absorbed in an organic material, on the mostbasic level, the electronic configuration of the target polymermolecules makes a transition to one of its excited electronic states.Each polymer is made of hundreds or more of sub-units of smallermolecules called monomers. The monomers are made of even smaller unitsof radicals consisting of combinations of hydrogen, carbon, oxygen, andnitrogen atoms. Depending on the energy level of the laser photons, apolymer can be broken into constituent monomers, radicals, or ionizedatoms. For a laser having a wavelength near 200 nm, a single laserphoton is not sufficiently energetic to break any molecular bond.However, after absorbing an initial photon, a molecule is promoted to anexcited electronic state configuration, with its electrons in higherenergy orbits.

[0125] With increased power levels of the laser beam, the excitedelectron density increases correspondingly. At the same time, theexcited electrons migrate down the polymeric chain of the organicmaterial, and spread towards the bulk volume with lower excited statedensity. The present invention recognizes that the excited stateelectronic orbitals are the means for energy storage that willeventually fuel the ablation process, and the electronic energy statemigration process plays a key role in the dynamics controlling theinitiation of the laser ablation.

[0126] As the laser beam intensity increases further towards theablation threshold, the excited electron density reaches a criticalvolume density such that the electronic orbitals can pair and transferthe sum of their energy to a single electron orbital. This processbreaks the molecule into two or more pieces, and releases an energeticelectron. At this point, the organic medium is damaged but not yetablated.

[0127] Consider now the geometric distribution of the excited stateorbitals in an organic material. As the laser light is absorbed in theorganic material, by Beer's law, the front surface where the material isfirst exposed encounters most of the laser photons, and the beamintensity decreases exponentially as it traverses deeper into thematerial. Hence, the spatial distribution of the excited state densityalso decreases accordingly, characteristic of the absorption coefficientof the material at the laser wavelength. It follows that the slope ofthe distribution curve of the excited state density is directly relatedto the absorption coefficient. Additionally, the steeper the slope ofthe excited state density distribution curve, the more spatiallylocalized is the excited state density. The preferred range of theabsorption depth of the surgical laser beam in the cornea is less thenabout 50 microns.

[0128] Alternatively, the ablation threshold can be reached at a lowerlaser peak power, provided that the material is exposed for a longerperiod. In accordance with the discussion above, if the total integratedenergy of a laser pulse is the same as that of a shorter pulse, theexcited state density established by the longer pulse would be lower dueto the additional time available for energy migration out of theirradiated volume. Therefore, to achieve the same ablation threshold fora longer pulse, the longer pulse must have a larger total integratedenergy than a shorter pulse having the same ablation threshold.Empirical results obtained from materials damage indicate that aparticular damage threshold can be reached with a pulsed laser beam 100times longer in duration than a shorter duration pulse, provided thatthe total integrated energy of the longer laser pulse is increased byabout 10 fold over the integrated energy of the shorter pulse.

[0129] In accordance with the discussion above, when using longerduration pulses, the energy migration process is counter-balanced byadditional laser beam pumping to build up the critical excited statedensity. Importantly, with a longer laser pulse, the excited stateorbitals diffuse from the front surface into the depth of the material(along the laser beam direction). Hence, the excited state distributioncurve will have less steep a slope compared to the curve from a shorterpulse. The present invention recognizes that the depth of the corneallayer which has sufficient excited state orbitals to satisfy the damagethreshold condition will be correspondingly deepened. Therefore, thecorneal damage inflicted by a longer duration laser pulse is moreextensive than the damage inflicted with a shorter duration pulse.

[0130] In consideration of these observations and characteristics, thepresent invention uses short duration laser pulses of about 1-5,000picoseconds to reduce inflicted damage to target tissues.

[0131] The other key objective of the present invention is to achieve ashallow yet reproducible etch depth at the cornea surface from eachlaser pulse. It is important to note that a reproducible etch depth willnot necessarily be attained at reduced levels of laser energy per pulse,especially when the energy level is close to being at an arbitrarilysmall value above the ablation energy threshold. For an excimer laser,the typical laser energy density in the surgical beam required forcornea ablation is about 150-250 mJ/cm². The ablation threshold levelfor excimer laser is at about 50 mJ/cm²; basically no ablative actioncan be observed at a laser energy density below this threshold level.

[0132] It is also important to note that observation of ablative actionnear the threshold condition is determined by a statistical process.That is, determination of the average etch depth for laser beam energiesnear the ablation energy threshold are derived by measuring actual etchdepth after hundreds or sometimes thousands of laser pulses over thesame location, and determining an average etch depth per pulse. On asingle shot basis, however, the etch depth could vary significantly, andmost of the laser pulses may not ablate any material at all.

[0133] Therefore, to ensure a reliable etch depth for each single laserpulse, the present invention recognizes that the operating energy perpulse has to be set at a multiple of the ablation energy thresholdlevel; a factor of 3 to 4 times the ablation energy threshold is usuallysufficient to achieve satisfactory results. Accordingly, the presentinvention uses an ablation energy density of less than or equal to about10 mJ/cm² to achieve a reproducible single-pulse etch rate of about 0.2microns or less per laser pulse, and preferably 0.05 microns or less perlaser pulse. This contrasts with current excimer lasers, which onlyprovide reproducible single-pulse etching at an etch rate of no lessthan about 0.3-0.5 microns per laser pulse, with consequent lightscattering due to cornea surface irregularities.

[0134] The present invention also recognizes the benefits of ablatingcornea with a laser beam having a low energy density. A gentle laserbeam, one that is capable of operating at a lower energy density for thesurgical procedures, will clearly have the advantage of inflicting lesstrauma to the underlying tissue. The importance of this point can beillustrated by considering the dynamics of the ablation process on amicroscopic scale: the ablation process is basically an explosive event.During ablation, organic materials are broken into their smallersub-units, which cumulate a large amount of kinetic energy and areejected out of the host surface at a supersonic velocity. The tissuebeneath the ablated region absorbs the recoil forces from suchejections. The present invention recognizes that a shallower etch depthinvolves less ejected mass per area, and hence reduces the recoil forcescorrespondingly. In accordance with the foregoing discussion, the lasercharacteristics of the present surgical system provide for an energydensity that results in a reproducible single-pulse etch rate of onlyabout 0.2 microns or less per pulse, and preferably about 0.05 micronsor less per pulse. Such a shallow etch rate means less mass ejected perlaser pulse. The damage impact on the underlying tissue is less by abouta factor of 10 in comparison with the lowest etch rate attainable in theprior art.

[0135] Another way to reduce the shock to the cornea is by using asmaller beam area at the cornea to reduce the integrated recoil forces.Consequently, the laser beam cross-sectional area of the inventionvaries from 1 mm in diameter to any tolerably achievable smallerdimension, as required by the particular type of surgery. Thischaracteristic of the invention contrasts with current excimer lasersurgical systems, which subject an ablation zone to a surgical beam thatis 4-6 mm in diameter.

[0136] In summary, the preferred laser corneal surgical system ablatescorneal tissue reproducibly at a single-pulse etch rate of about 0.2microns or less per laser pulse, and preferably about 0.05 microns orless per laser pulse. In accordance with the present invention, a lasersource with a wavelength range of about 198-300 nm (with a preferredrange of about 198-215 nm), and a pulse duration of about 1-5,000picoseconds, achieves reliable single pulse ablation on the cornea. Theintensity of the laser pulses is regulated to have an ablation energydensity of less than or equal to about 10 mJ/cm².

[0137] The Inventive Apparatus

[0138]FIG. 1 shows the preferred configuration of the inventiveapparatus. A laser unit 100 generates an initial laser beam B1. Thelaser unit 100 is of the type that can output a beam rapidly deflectableor scannable under electronic control in two dimensions to any locationin an area defined by orthogonal X and Y axes. One such laser unit isdescribed in detail in the co-pending, commonly-owned patent applicationfor invention entitled “Two Dimensional Scanner-Amplifier Laser” (U.S.patent application Ser. No. 07/740,004), and in the pertinent textreproduced below.

[0139] The initial laser beam B1 comprises a sequence of laser pulseshaving a pulse repetition rate of about 100 to 50,000 pulses per second.Each laser pulse has a pulse duration which can be varied from 1picosecond to about 5,000 picoseconds. The actual number of laser pulsesused for a surgery is determined by the amount of tissue to be removed.

[0140] In a preferred embodiment, the laser unit 100 includes a seedlaser 102 and a scanner-amplifier laser 104. Preferably, the laser mediain both the seed laser 102 and the scanner-amplifier 104 is a Ti-dopedAl₂O₃ solid state laser crystal. Further details of the structure andoperation of the laser unit 100 are set forth below.

[0141] After emerging from the laser unit 100, the laser beam B1 passesthrough a computer-controllable, motorized zoom lens 106, which providescontrol over the diameter of the laser beam B1. In practice, the zoomlens 106 may be placed in a number of suitable positions along theoptical path of the laser beam between the laser unit 100 and a target.The motor actuation of the zoom lens 106 may be by any known means, suchas electrical gear drives or piezoelectric actuators.

[0142] The preferred laser wavelength for the initial laser beam B1 isin the range of about 790-860 nm. The laser photon energy in the initiallaser beam B1 is then converted in a first wavelength converter 108(described below) by nonlinear wave mixing to a second laser beam B2having approximately twice the initial laser beam photon energy, and awavelength in the range of about 395-430 nm.

[0143] To attain the preferred operating laser wavelengths of about198-215 nm, the second laser beam B2 is passed through a secondwavelength converter 110 (described below). The laser photon energy inthe second laser beam B2 is again converted by nonlinear wave mixing toa third laser beam B3 having approximately four times the initial laserbeam photon energy, and a wavelength in the range of about 198-215 nm.

[0144] In an alternative embodiment, the initial laser beam B1 may bewavelength converted to the desired wavelength range of about 198-215 nmusing a one-step converter (described below).

[0145] Surgical Laser Beam Control System

[0146] While the third laser beam B3 could be used directly for surgicalpurposes, in the preferred embodiment, the entire surgical laserapparatus includes a number of control and safety systems. Inparticular, the present invention includes means for monitoring andcontrolling the intensity of the beam, means for blocking the surgicalbeam in the event of a malfunction, means for monitoring and controllingthe laser beam diameter and intensity profile, and means for verifyingthe two-dimensional (X-Y) scan position of the surgical beam.

[0147] Referring again to FIG. 1, the third laser beam B3 passes througha beam intensity controller 112, the output of which is the surgicallaser beam S. The beam intensity controller 112 permits regulation ofthe energy of each laser pulse so that the etch depth of each pulse maybe precisely controlled.

[0148] In the preferred embodiment, the beam intensity controller 112 isan electro-optical filter, such as an electrically activated Pockelscell in combination with an adjacent polarizing filter. The Pockels cellmay include, for example, LiNbO₃, or any other electro-optical crystal,such as potassium dihydrogen phosphate (KH₂PO₄), also known as KDP.Pockels cells are commercially available from several sources, includingMedox Electro-Optics of Ann Arbor, Mich. With the application ofelectric voltage across the electro-optical crystal in a Pockels cell,up to a half-wave retardation in the electric field vector of theincident laser beam can be generated. Depending on applied electricalvoltage, the linear polarization of a laser beam traversing the crystalcan be retarded from a horizontal polarization to vertical, or viceversa. The polarizer placed adjacent the Pockels cell acts as a selectorwith respect to the incident beam from the Pockels cell. As is known, ifthe beam impinging on the polarizer is orthogonally polarized by thePockels cell, the beam will be essentially blocked by the polarizer.Lesser degrees of retardation generated by the Pockels cell will resultin some of the light passing through the polarizer. By controlling theamount of retardation generated in the Pockels cell, the intensity ofthe incident laser beam can be electrically controlled.

[0149] In the preferred embodiment, the beam intensity controller 112 iscoupled to a computer control unit 114, which is suitably programmed tovary the intensity of the output surgical laser beam S as required for aparticular surgical procedure. The degree of retardation as a functionof applied electrical signal can be ascertained by standard calibrationtechniques. The preferred location of the beam intensity control unit112 is as shown in FIG. 1. However, the beam intensity control unit 112can be placed at several suitable locations in the beam path between thelaser unit 100 and a target. In the preferred embodiment, the intensityof the surgical beam S is regulated to have an ablation energy densityof less than or equal to about 10 mJ/cm².

[0150] The present invention optionally provides for positive feed-backmeasurement of the beam intensity. A partially transmissivebeam-splitting mirror 116 is placed after the beam intensity controller112, and the reflected beam R_(i) is directed to a beam intensity sensor118. The beam intensity sensor 118 may be simply a photocell, althoughother elements, such as focussing optics, may be included. By monitoringthe electrical output of the beam intensity sensor 118 with the computercontrol unit 114, the intensity of the surgical laser beam S can bepositively measured to verify the proper operation of the beam intensitycontroller 112. The output of the beam intensity sensor 118 as afunction of intensity of the surgical laser beam S can be ascertained bystandard calibration techniques.

[0151] The inventive system also preferably includes a safety shutter120, which is coupled to the computer control unit 114. The safetyshutter 120 may be, for example, a mechanically-actuated shutteroperated in a “fail-safe” mode. For example, the safety shutter 120 mayinclude a solenoid-actuated shield that is positively held open byapplication of electrical energy to the solenoid. Upon command of thecomputer control unit 114, or failure of the entire system, electricalenergy to the solenoid is cut off, causing the solenoid to retract theshield into position to block the path of the surgical laser beam S.

[0152] Alternatively, the safety shutter 120 may include a Pockels celland polarizer configured as a light valve, with the Pockels cell biasedwith respect to the polarizer by application of an electrical voltagesuch that maximum light is normally transmitted by the combination.Cessation of the applied voltage will cause the output of the Pockelscell to become polarized orthogonal to the transmission direction of thepolarizer, hence blocking the surgical laser beam S. Using thisalternative configuration, the safety shutter 120 and the beam intensitycontroller 112 may be combined into a single unit.

[0153] Any other suitable means for quickly blocking the surgical laserbeam S on command or in the event of system failure may be used toimplement the safety shutter 120. In practice, the safety shutter 120may be placed in a number of suitable positions along the optical pathof the laser beam between the laser unit 100 and a target.

[0154] To control beam diameter, the inventive system provides apartially transmissive beam-splitting mirror 122 that reflects part ofthe beam R_(d) to a beam diameter sensor 124. In practice, the beamdiameter sensor 124 may be placed in a number of suitable positionsalong the optical path of the laser beam between the laser unit 100 anda target.

[0155] Referring to FIG. 2A, the beam diameter sensor 124 preferablyincludes at least a diverging (concave) lens 200 and a converging(convex) lens 202 configured as a magnifying telescope (i.e., the twolenses have a common focal point, with the focal length f₂ of theconverging lens 202 being greater than the focal length f₁ of thediverging lens 200, and having optical centers aligned with the incidentlaser beam in its undeflected position). The incident beam R_(d) entersthe diverging lens 200 and exits the converging lens 202. Such aconfiguration of lenses, while enlarging the incident beam, will alsoreduce the scan angle of the exiting beam.

[0156] The resulting enlarged beam is directed to a low density, lowcontrast imaging device 204, such as a charge-coupled device (CCD)camera. In the preferred embodiment, a CCD camera-having a 64×64 pixelarray with two or more bits of contrast is suitable. Such cameras areavailable commercially. The two lens 200, 202 are chosen to expand theincident beam R_(d) so that the largest possible diameter 206 for thebeam just fits within the imaging device 204 (see FIG. 2B, which showsonly one row and one column of pixels).

[0157] In the preferred embodiment, the size of the beam is determinedby periodically addressing a central row and a central column of theimaging device 204 and counting the number of pixels on each sampledaxis that have been illuminated. By comparing the diameter of the beamin both the X and Y directions, the beam diameter sensor 124 candetermine whether the incident laser beam B1 is approximately circularand has the desired diameter. For example, if the number of pixelsilluminated on each axis is 20 pixels, the beam will be known to havehalf the diameter of a beam that illuminated 40 pixels along both axes.As another example, if for any reason the beam has become elliptical,the number of pixels of the imaging device 204 illuminated along theX-axis will differ from the number of pixels illuminated along theY-axis.

[0158] The beam diameter sensor 124 can also be used to determine theintensity profile of the laser pulses, since each pixel in the beamdiameter sensor 124 can generate an output indicative of the intensityof light incident to the pixel. By comparing pixel values from radiallysymmetric points in the pixel array, it can be determined if an incidentlaser pulse or series of pulses has the desired radially symmetricintensity profile, or if the pulses have developed “hot spots” ofout-range intensity values.

[0159] The output of the beam diameter sensor 124 is coupled to thecomputer control unit 114. The computer control unit 114 is in turncoupled to the motorized zoom lens 106, which provides control over thediameter of the laser beam B1. The computer control unit 114 is suitablyprogrammed to vary the diameter of the laser beam as required for aparticular surgical procedure. The output of the beam diameter sensor124 as a function of beam diameter can be ascertained by standardcalibration techniques.

[0160] This configuration provides positive feed-back of the beamdiameter emanating from the laser unit 100. If the beam diameter sensor124 detects an out-of-range beam (either diameter or intensity profile),the computer control unit 114 can take appropriate action, includingactivation of the safety shutter 120.

[0161] To verify the X-Y scan position of the laser beam, the inventivesystem provides a partially transmissive beam-splitting mirror 126 thatreflects part of the beam energy R_(l) to a beam location sensor 128.Referring to FIG. 3A, the beam location sensor 128 preferably includesat least a converging (convex) lens 300 and a diverging (concave) lens302 configured as a reducing telescope (i.e., the two lenses have acommon focal point, with the focal length f₂ of the diverging lens 302being greater than the focal length f₁ of the converging lens 300, andhaving optical centers aligned with the incident laser beam in itsun-deflected position). The incident beam R_(l) enters the converginglens 300 and exits the diverging lens 302. Such a configuration oflenses, while reducing the incident beam, will also increase the scanangle of the exiting beam.

[0162] The resulting increased-scan angle beam is directed to a siliconphoto-detector 304 which provides a voltage reading with respect to thetwo-dimensional (X-Y) location of an illuminating spot at the detectorsurface. Such detectors are commercially available from a variety ofsources, including United Detector Technologies, UDT Sensors, Hawthorne,Calif. The output of the beam location sensor 128 is coupled to thecomputer control unit 114.

[0163] Calibration of the voltage reading generated from theun-deflected incident beam position on the detector 304 will indicatethe origin OR of the laser beam in the XY-scan plane. Any deflection ofthe beam from the origin OR will generate voltage readings indicative ofthe spot on the detector 304 surface illuminated by the laser beam.These voltage readings are calibrated against the indicated location ofthe surgical beam as set by the computer control unit 114. Duringoperation, the output of the beam location sensor 128 would be sampledperiodically (for example, about 1,000 times per second) and compared toa prepared calibration table in the computer control unit 114 todetermine if the actual beam position matches the indicated position.

[0164] This configuration provides positive feed-back of the beamposition emanating from the laser unit 100. If the beam location sensor128 detects an out-of-position beam, the computer control unit 114 cantake appropriate action, including activation of the safety shutter 120.

[0165] Thus, the preferred embodiment of the inventive surgical laserapparatus provides for safe and effective surgery by continuouslymonitoring all aspects of the condition of the surgical laser beam S,including beam intensity, diameter, and X-Y scan position.

[0166] Eye Tracking System

[0167] When using the inventive system, it is important to minimize eyemovement with respect to the surgical laser beam S. Therefore, in orderto locate the eye relative to the surgical laser beam S, a conventionalsuction ring 400, such as is shown in FIG. 4A, is used to immobilize theeye. Such devices are commercially available, for example, from SteinwayInstruments of San Diego, Calif. Such suction rings are furtherdescribed, for example, in U.S. Pat. No. 4,718,418 to L'Esperance, Jr.

[0168] A suction ring 400 is normally applied to the white (sclera)region of the eye and connected to a low suction pressure sufficient toclamp the ring 400 to the eye, but not so great that the cornea isdistorted. The use of such a ring 400 is well-known in the art.

[0169] Despite the use of a suction ring 400 to immobilize an eye, somemovement of the eye may occur (possibly through movement of the suctionring 400 itself by a surgeon). Therefore, the present invention providesan eye tracking system 130 to compensate for relative movement betweenthe eye and the surgical laser beam S. As shown in FIG. 1, the eyetracking system 130 is placed in the path of the surgical laser beam S,preferably in close proximity to a target eye.

[0170] Referring to FIGS. 4A, 4B, and 4C, a conventional suction ring400 is provided with distinct marks 402, 404, 406 on the back of thering facing the surgical laser system (see particularly FIG. 48). Themarks may or may not be subdivided by cross-marks, for visual referenceby a surgeon. In the preferred embodiment, the marks include an X-axis402, an orthogonal Y-axis 404, and a radial axis 406. The marks arepreferably made to be highly reflective of broadband illuminating light,and the background of the suction ring 400 is preferably flat black toenhance contrast and minimize extraneous reflections.

[0171] The eye tracking system 130 includes a pair of steering mirrors408, 410 each comprising a reflector mounted on a galvanometer scanneror similar actuator device which is controllable by a computer, and arotational control device consisting of a dove prism 409 mounted withits rotational axes aligned with the surgical laser beam S (see FIG.4C). A motorized drive unit 411 is attached to a gear or bell drivedesigned to control the rotation of the dove prism 409. As is known,rotation of a dove prism will cause an exit beam to be rotated withrespect to an incident beam. The steering mirrors 408, 410 are mountedwith their rotational axis orthogonal to each other and situated suchthat the surgical laser beam S enters the eye tracking system 130,passes through the dove prism 409, bounces off of a first steeringmirror 408 to the second steering mirror 410, and thence to a targetcornea. The steering mirrors 408, 410 and the dove prism 409 thereforeprovide a means to “bias” the surgical laser beam S to compensate formovement of the eye relative to the surgical laser beam S.

[0172] Control of the steering mirrors 408, 410 and the dove prism 409is provided by reflecting the image of the illuminated marks 402, 404,406 on the suction ring 400 back up the optical path of the surgicallaser beam S to a partially transmissive beam-splitting mirror 412,which directs the reflected image onto a tracking sensor 414 (otherelements, such as focussing optics, may be included in the trackingsensor 414). In the preferred embodiment, the tracking sensor 414includes three linear array sensors 416, 418, 420. Each linear arraysensor 416, 418, 420 corresponds to one of the marks 402, 404, 406 onthe suction ring 400, and is oriented orthogonally to the correspondingmark. In the preferred embodiment, each linear array sensor may be alinear reaction with about 1,024 or more sensing elements per inch. Suchlinear reticons are available commercially, such as from EG&G,Princeton, N.J.

[0173] Because of the orthogonal orientation of each mark 402, 404, 406with respect to a corresponding linear array sensor 416, 418, 420, anymovement of the suction ring 400 will result in a relative displacementof the reflected image of one or more of the marks 402, 404, 406 withrespect to the corresponding linear array sensor 416, 418, 420. Suchmovement can be easily detected by comparing a stored initial positionof each mark 402, 404, 406 with the position of each mark determined byperiodically scanning the output of each linear array sensor 416, 418,420. Because of the relative orientations of the marks 402, 404, 406,translational movements of the suction ring 400 in the X and Ydirections, as well as rotational movements, can be detected. The outputof the tracking sensor 414 as a function of the positions of thereflected marks 402, 404, 406 can be ascertained by standard calibrationtechniques.

[0174] The eye tracking system 130 may be provided with its own feedbackcontrol system to adjust the positions of the steering mirrors 408, 410and the dove prism 409 to compensate for detected relative motion of theeye with respect to the surgical laser beam S. Alternatively, the eyetracking system 130 may be coupled to the computer control unit 114.Control of the eye tracking system 130 through the computer control unit114 is preferred, since the computer control unit 114 can activate thesafety features of the inventive system (e.g., the safety shutter 120)if the target eye is improperly aligned with the surgical laser beam Sor if a failure occurs in the eye tracking system 130.

[0175] In either case, the output of the tracking sensor 414 would bemonitored, and the positions of the steering mirrors 408, 410 and thedove prism 409 adjusted accordingly. In compensating for relative eyemovement, it is preferable to first correct for the reflected imageposition of the mark 402, 404, 406 having the greatest deviation.

[0176] The inventive eye tracking system thus provides a means ofimproving the accurate placement of laser pulses to the cornea. By usingdistinct marks 402, 404, 406 on the suction ring 400, the inventionprovides more precise detection of relative movement of the eye comparedwith systems using a natural indicator, such as the pupil or the scleraof the eye (which, in any case, could not indicate rotational movement).

[0177] Method of Depositing Laser Pulses

[0178] Another problem addressed and solved by the present invention isthe proper deposition of laser beam energy on the cornea to ablatetissue to any desired depth while leaving an optically-smooth corneasurface after the laser surgery. In the prior art, it has been known toapply a laser beam in a raster scan or a circular or spiral scan patternover the area of the cornea where tissue is to be removed (see, forexample, FIGS. 3 and 4 of U.S. Pat. No. 4,718,418 to L'Esperance, Jr.).A problem with such patterns when used with prior art laser systems isthat such systems ablate tissue to a depth of about 0.3 to 15 microns ormore per laser pulse. A typical procedure for laser etching of thecornea must remove from about 0.2 microns or less, up to about 50microns of tissue. Since it is essentially impossible to accuratelyplace each and every pulse so that it is perfectly contiguous to aneighboring pulse, ridges or grooves in the corneal surface of the samemagnitude will result from the imperfect pattern of deposition of laserpulses. Accordingly, post-operative visual acuity will be reducedbecause of light scattering from the inhomogeneity of the tissue at theuneven interface.

[0179] Another problem of the prior art, particularly with excimerlasers, is that the beam intensities used have principally had a “tophat” intensity profile of the type shown in FIG. 6A. Such an intensityprofile will result in essentially a mirror-image ablation profile, asshown in FIG. 68.

[0180] Attempts have been made to avoid the sharp edges caused by a “tophat” intensity profile by adopting instead a radial profile having aGaussian intensity profile (intensity=e^(−2(r/ω)) ² , where ω is thebeam waist) (curve 600 in FIG. 6C) or a super-Gaussian intensityprofile, which is a slightly modified Gaussian curve with a lessergradient at the center (curve 602 in FIG. 6C), resulting incorrespondingly shaped tissue ablation profiles, as shown in FIG. 60.

[0181] To overcome the problem of haze-inducing ridges and grooves, theprior art has attempted to overlap Gaussian or super-Gaussian beamintensity profiles to generate a smoother average etch profile. Forexample, as shown in FIG. 6E, overlapped Gaussian beam intensitiesresult in an average beam intensity equivalent to that shown in FIG. 6F,which results in a corresponding mirror-image ablation etch profile asshown in FIG. 6G.

[0182] A problem with this approach is that, during photoablation,vaporized tissue material is expelled from each tissue site ablated by alaser pulse (the expelled tissue is known as “plume”). It is known thatsuch expelled debris can scatter photons in an incoming laser beam (thisis known as “shadowing”). As should be expected, this phenomena reducesthe intensity of the beam. Thus, when laser pulses are overlapped asdescribed above, a prior adjacent pulse will generate a plume thatpartially shadows or obscures the incoming laser beam of a subsequentadjacent laser pulse, causing non-uniformity of tissue ablation andhence irregularities on the cornea surface.

[0183] An additional problem of prior art overlapped laser depositionpatterns is that such patterns are repeated in such a manner thatsignificant-sized ridges and grooves are still formed between pulsecenters.

[0184] The solution of the present invention to the problems of theprior art laser deposition patterns is to use a Gaussian, or,preferably, a super-aussian intensity profile for each laser pulse, andto deposit the pulses in a plurality of layers, each layer having aregular geometric pattern. The origin of each layer of the pattern isoff-set by a specific distance in either the X or Y dimension from eachprior, or subjacent, layer. The inventive pattern avoids the problem ofplume by not overlapping the laser pulses of any one layer, andovercomes the problems of prior art ridge and groove formation byuniformly depositing laser energy over the surface to be etched. Becauseof the shallow etch depth of each laser pulse of the present invention(about 0.2 microns or less), etching can be stopped essentially at anypoint in the ablative process while leaving an optically smooth corneasurface.

[0185] Referring to FIG. 7A, shown is a part of a first level scanpattern, comprising a single etch layer. In the preferred embodiment,each laser pulse creates an etch profile with an approximately circularcross-section with a radius r, which may typically range from about 0.02mm to about 0.5 mm. The scan pattern programmed into the laser unit 100lays down a pattern of pulses in a hexagonally-packed array of the typeshown in FIG. 7A. That is, the center A of each circular cross-sectionetch circle 700 of radius r is spaced a distance D, equal to 2r, fromthe center A of each other etch circle 700. As is known, the -patternresulting from this simple criteria is a hexagonally-packed array ofcircles (the dotted hexagons shown in FIG. 7A are for purposes ofillustrating the packing pattern, and do not form any part of the etchprofile).

[0186] While it is preferred that the etch circles be non-overlappingand contiguous, the invention encompasses slight overlapping and/orspacing of etch circles due to tolerance limits on positioning etchcircles with a practical laser apparatus.

[0187] A benefit of the hexagonally-packed array of etch circles is thatthe pattern is simple to program into the scanning control system of thelaser unit 100 as a modified raster scan. If etch circle 702 havingcenter A′ is considered to be the origin for the initial first levelpattern, the laser unit 100 need only move the laser beam in the Xdirection a distance of D to the center A for the next etch circle 704.Additional etch circles are created in the same manner for the firstrow, until the opposite edge of the area to be ablated is reached. Suchprecision of placement of etch circles is made possible by the highlyaccurate X-Y positioning capability of the laser unit 100, particularlywhen used in conjunction with the eye-tracking system described above.

[0188] After the first row of etch circles is completed in the samemanner, the laser beam is moved down in the Y direction a distance ofabout 0.866D (one-half the square root of 3 times D, representing thevertical distance between centers of adjacent rows), and left or rightalong the X direction by ½D (representing the horizontal distancebetween centers of adjacent rows). The beam is then either scannedbackwards, or returned to the original “edge” of scanning and scannedforwards in the same manner as the first row. Each subsequent row iscreated in the same manner, until the bottom edge of the area to beablated is reached, thus completing the first level layer.

[0189] Although a regular order of etch circle deposition is preferredfor ease of programming the laser unit 100, the accurate X-Y positioningcapability of the laser unit 100 permits the etch circles for aparticular layer to be deposited in any order, including randomly.

[0190] A characteristic of the first level pattern shown in FIG. 7A isthat no circular etch substantially overlaps any other circular etch.Consequently, the problem of plume is minimized. While laying down onlythe first level pattern shown will result in ridges in the gaps betweenetch circles, because of the shallow etch depth used, thecrest-to-trough distance of any ridge area R to the center A of any etchcircle 700 will be at most about the same as the etch depth of a singleetch (about 0.2 microns or less).

[0191] After laying down the initial first level pattern shown in FIG.7A, the inventive method preferably lays down a second level pattern,comprising three etch layers. Each second level etch layer is an exactreplica of the single etch layer of the first level pattern (i.e., anhexagonally-packed array of etch circles of radius r). However, theorigin of each of the three layers with respect to each other and to thefirst level layer is unique. In order to minimize ridges and grooves inthe etched cornea, each layer of the second level pattern is offset fromthe single layer of the first level pattern to even-out the distributionof laser energy across the cornea. This concept of off-settingsubsequent layers in exact relationship with respect to an initial layeris in contrast to the prior art, which typically repeats the etchingprocess by sweeping the laser beam across the ablation zone withoutreference to the exact location of each of the laser pulses.

[0192] More specifically, the origin of the first layer of the secondlevel is set at point B1 (or an equivalent point; see below) of FIG. 7A,which is one-half the distance D between the first level origin A′ ofetch circle 702 and the center of the adjacent etch circle 704. Usingpoint B1 as an origin, the laser unit 100 is programmed to lay down anentire array of etch circles covering the area to be ablated, using thesame rules for changing beam location as described above for the firstlevel etch layer.

[0193] Similarly, the origin of the second layer of the second level isset at point B2 (or an equivalent point; see below) of FIG. 7A, which isone-half the distance D between the first level origin A′ of etch circle702 and the center of the adjacent etch circle 706 in the next row.Using point B2 as an origin, the laser unit 100 is programmed to laydown an entire array of etch circles covering the area to be ablated,using the same rules for changing beam location as described above forthe first level etch layer.

[0194] Lastly, the origin of the third layer of the second level is setat point B3 (or an equivalent point; see below) of FIG. 7A, which isone-half the distance D between the center of etch circle 704 and thecenter of the adjacent etch circle 706 in the next row. Using point B3as an origin, the laser unit 100 is programmed to lay down an entirearray of etch circles covering the area to be ablated, using the samerules for changing beam location as described above for the first leveletch layer.

[0195] The resulting etch pattern for the second level will resemble thepattern shown in FIG. 7B, which shows the first level centered at A′ asthick-lined circles 715, the first layer centered at point B1 as solidcircles 716, the second layer centered at point B2 as dotted circles717, and the third layer centered at point B3 as dashed circles 718. Ifdesired, the level one and two etch patterns may be repeated as neededto obtain the desired amount of ablation.

[0196] Although the second level comprises three layers, all threelayers need-not be completed. Further, the first layer of the secondlevel can be started with its origin at any of the three points B1, B2,or B3, since all of these points are geometrically equivalent. Moregenerally, equivalents of these three offset points exist throughout thegrid of centers A defined by the initial first level layer. Thus, anyequivalent offset point in the second level may be selected as theorigin of one of the three layers comprising the second level. Theoverall etch profile (determined as described below) determines which ofthe equivalent second level offset points will be selected to achievemaximum ablation where required in the surface being etched, and whetherthe desired degree of smoothness of finish requires completion of eachof the second level layers. However, to ensure evenness of etching, itis generally desirable to complete all layers of the second level beforeadditional levels of etching commence.

[0197] As an alternative way of modelling the first and second levels,they may instead be considered as a single etch pattern “unit”comprising four etch layers arranged to overlap in the manner shown inFIG. 7B.

[0198] If desired, further levels of etch patterns could be generated ina similar fashion by repeating levels one and two, using new origins. Acharacteristic of the geometry of an hexagonal packing array lends isthat it lends itself to creation of repeating regular patterns. Forexample, referring to FIG. 7A, the origins B1, B2, and B3 for the secondlevel etch layers comprise the midpoints of a triangle T1 connecting thecenters of etch circles 702, 704, and 706. A second triangle T2 can becreated by connecting the centers of etch circles 704, 706, and 708(shown in dotted outline in FIG. 7A). These two triangles comprise asymmetrical unit that is repeated throughout the pattern of centers Adefined by the initial first level pattern. Moreover, by connectingadjacent midpoints, each of the triangles T1 and T2 can be subdividedinto four smaller, equal-sized triangles, as shown in FIG. 7A. Themidpoints of each sub-triangle in a T1-T2 unit not shared with a similarT1-T2 type unit comprise twelve offset points C1-C12 that haveequivalents throughout the grid of centers A defined by the initialfirst level layer.

[0199] Each of these equivalent offset points C1-C12 can be used as anorigin for a set of level one and level two etch patterns. That is,taking point C11 as an example, C11 can be selected as the center of alevel one pattern. The level one pattern centered at C11 then defines anew grid for a corresponding level two pattern. Similarly, point C2could then be selected as the center of another level one pattern. Thelevel one pattern centered at C2 then defines a new grid for acorresponding level two pattern.

[0200] With such third level equivalent offset points defined, any ofthem may be selected as the origin of one of the 48 layers (12 levelone/level two sets) comprising the third level. The overall etch profile(determined as described below) determines which of the equivalent thirdlevel offset points will be selected to achieve maximum ablation whererequired in the surface being etched, and whether the desired degree ofsmoothness of finish requires completion of each of the third levellayers. However, to ensure evenness of etching, it is generallydesirable to complete all layers of the third level before additionallevels of etching commence.

[0201] The process of defining subsequent levels may be extended asnecessary, by defining equivalent offset points based on repeatinggeometrical units determined by the grid of centers A defined by theinitial first level layer. The general rule is to divide a previouslevel into additional triangles based on the grid defined by the initialfirst level layer. This is done by connecting three adjacent origins toform such triangles, and then using the midpoints of such triangles asnew origins.

[0202] The extent of improvement on the surface smoothness by theprecise positioning of multiple layers of etch profile is illustrated inthe following: Using a Gaussian laser beam profile as an example, andsetting the laser energy density at the peak of the laser pulse to befour times the ablation threshold, the surface smoothness can becharacterized in relation to the maximum etch depth of a single laserpulse. For example, after applying the first level etch pattern asdescribed above using a Gaussian intensity profile, the maximumcrest-to-trough distance of the etch patterns anywhere within theboundaries of the etched area will of course be 100% of the maximumcrest-to-trough distance of a single etch circle. By applying just twolevels of etch patterns as described above using a Gaussian intensityprofile, the maximum crest-to-trough distance of the overlapped etchpatterns anywhere within the boundaries of the etched area will be atmost about 53% of the initial first level pattern alone. By applyingthree levels of etch patterns using a Gaussian intensity profile, themaximum crest-to-trough distance of the overlapped etch patternsanywhere within the boundaries of the etched area will be at most about20% of the initial first level pattern alone. Since the crest-to-troughdistance for the initial first level pattern is about 0.2 microns orless, and preferably about 0.05 microns or less, even the second levelpattern may be sufficient to achieve the desired result.

[0203] This analysis is graphically presented in FIG. 7D, which shows aset of graphs showing the cumulative crest-to-trough distances of levelone, level two, and level three etch patterns in accordance with thepresent invention. The Y-axis of each graph shows the cumulative etchdepth in units of the maximum etch depth of a single laser pulse. TheX-axis shows etch depth as a function of the distance from an etchcircle center out along one of the three symmetry axes for an hexagonalarray. FIG. 7E shows the measurement axes used to compute the level twoand level three crest-to-trough distances of FIG. 7D. The notation forthe endpoints of the X-axis of FIG. 7D corresponds to the notation forthe measurement points shown in FIG. 7E.

[0204] The regular characteristics of the inventive deposition systemare useful when etching the cornea in a “stepped pyramid” fashion (interms of laser pulse count), with fewer etch circles deposited towardsthe periphery of the cornea and more etch circles deposited towards thecenter. As shown in FIG. 7C, the resulting overall etch patterntypically resembles concentric circles (although other shapes arepossible). When etching from the outer edge to the center, the entirecornea is etched to the diameter of ring 720, using the etch patternsdiscussed above. Using the original grid of centers A from the veryfirst level, a new origin at an equivalent offset point within ring 722is chosen when the tissue in ring 720 has been etched to the desireddegree. Etching continues over the entire cornea encompassed within thediameter of ring 722. Again, using the original grid of centers A fromthe very first level, a new origin at an equivalent offset point withinring 724 is chosen when the tissue in ring 722 has been etched to thedesired degree. Etching continues over the entire cornea encompassedwithin the diameter of ring 724. The process continues in similarfashion until the center ring 726 is etched to the proper depth. (Notethat the diameter of the laser pulses may be made smaller in the innerrings to provide a finer etching grid, in which case, a new initialfirst level pattern defining such a grid may be laid down and used indetermining equivalent offset points for subsequent levels).

[0205] As should be clear by considering FIG. 7C, the etch process couldbe done in reverse order, with center ring 726 etched first, then thearea encompassed within the diameter of ring 724, then the areaencompassed within the diameter of ring 722, and finally the areaencompassed within the diameter of ring 720.

[0206] Wavelength Converter Means

[0207] As described above, the inventive system includes at least onewavelength converter to alter the wavelength of the initial laser beamB1 to the desired wavelength in the range of about 198-215 nm.

[0208] A first example of one wavelength converter is shown in FIG. 5A.In FIG. 5A, the initial laser beam B1 emerging from the laser unit 100is shown to have been scanned at an incident angle θ₁ from its centralposition 500, which is defined as the center position of the total scanangle to be covered for an intended surgical operation. Generally, thelaser beam is scanned in two dimensions, and hence two angular positionsare needed to specify each unique beam position. In the preferredembodiments shown in FIGS. 2A and 2B, the optical system is sphericallysymmetric. Thus, only one of the incident scan angles will beillustrated in the following discussion without loss of generality.

[0209] In FIG. 5A, a convex lens A is located at a distance f(A), thefocal length of lens A from the pivot point of the scanned laser beamB1. In the illustrated embodiment, the pivot point is inside thescanner-amplifier unit 104, at an equivalent position of the scan mirrornear the exit dielectric mirror, as described below. A nonlinear opticalcrystal 502 is chosen such that phase matching angles exist with aproper crystal orientation so that a fundamental laser wavelength withina range of about 790-860 nm can be converted to its second harmonic at awavelength in the range of about 395-430 nm. One possible such crystalis beta-Ba₂BO₄ (beta barium borate). This crystal has a phase matchingangle at about 26-30° for the wavelength range stated above, in type Iphase matching conditions. The nonlinear crystal 502 is positioned at adistance f(A) from the lens A. The incident laser beam B1 is weaklyfocused at the crystal 502 with a choice of a long focal length for lensA. Another convex lens 8 located at the focal length f(B) of lens B fromthe crystal 502 recollimates the beam into an emergent laser beam B2.Preferably, both lenses A and B are coated for maximum transmission atlaser lengths for which each transmits.

[0210] The dimensions of the nonlinear crystal 502 are chosen such thatthe surface area where the incident laser beam B1 enters the crystal 502is sufficiently large that the laser beam will not be cut off at theextremity of its scan angles. The length l of the crystal 502 is suchthat the conversion efficiency is to be optimized, in consideration ofthe walk off between the fundamental and the second harmonic beam, thegroup velocity dispersion, and the spectral bandwidth for the shortduration laser pulses. The entrance surface of the nonlinear crystal 502is coated for maximum transmission of the fundamental wavelengths andthe exit surface is coated for maximum transmission of the secondharmonic wavelengths.

[0211] The optical arrangement of the present embodiment of thewavelength converter offers several additional advantages: the scanangle η₁ of the incident laser beam B1 can be magnified or reduced bychoosing the proper focal length for the lens B. If f(B) is smaller thanf(A), the beam scan angle θ₂ in FIG. 5A will be magnified by a factorf(A)/f(B). On the other hand, if f(B) is larger than f(A), the beam scanangle θ₂ will be reduced by a factor of f(A)/f(B).

[0212] It is important to note that the laser beam, which subtends anangle θ₁ from the central position 500, becomes parallel-to-the centralposition 500 after passing through lens A. Therefore, lens A providestwo improvements in the harmonic conversion process: the laser photondensity at the non-linear crystal 502 is increased due to the smallerbeam area, and the laser beam orientation incident at the non-linearcrystal 502 is maintained at all scan angles, thereby maintaining thephase matching conditions of the beam while it is being scanned.

[0213] Another advantage of the embodiment shown in FIG. 5A results fromthe change of location of the incident laser beam B1 through thenonlinear crystal 502 as the beam is scanned. Within the nonlinearcrystal 502, a small amount of the laser beam is absorbed, resulting ina thermal gradient across the beam cross-section. This temperaturevariation at different portion of the incident beam B1 degrades thephase matching conditions, and places a limit on the conversionefficiency of the harmonic generation process. By moving the beam overan area during scanning, the thermal energy is effectively distributedover that area, and the average power loading in the crystal 502 iseffectively reduced. If the area is sufficiently large, the laser pulsesbecomes non-overlapping. Reduction of pulse overlapping also results inan improved crystal damage threshold. For a laser beam with a highrepetition pulse rate, if the laser beam is stationary, as in the priorart, there is a time delay requirement, so that the effect of a laserpulse through a nonlinear crystal is allowed to dissipate before thenext laser pulse arrives. This requirement places an upper limit on therepetition rate at about 10,000 pulses per second. The present inventionovercomes the above prior art limitations, and provides an improvedmethod of laser wavelength conversion to attain a higher conversionefficiency and a higher crystal damage threshold by scanning the laserbeam across the nonlinear crystal 502. With the present invention, therepetition rate of the surgical beam can be extended to over 50,000pulses per second.

[0214] A second example of a wavelength converter is shown in FIG. 5B.In this embodiment, lens A and the nonlinear crystal 502 are similarlylocated as specified for FIG. 5A, except that the crystal 502 ispositioned slightly closer to lens A. A high reflective mirror 504 islocated at a distance f(A) from lens A. The reflective mirror 504 hasthe characteristics of being highly reflective at the fundamental andthe second harmonic wavelengths of the incident laser beam B1. Apartially-transmissive beam-directing mirror 506 is included in the beampath, and is set at about 45° from the laser beam central position 500.The beam-directing mirror 506 is coated with dielectric thin films forhigh transmission of the fundamental wavelength, and high reflection ofthe second harmonic wavelength at near 45°. An incident laser beam B1 atan angle θ₁ with the central position 500 passes through thebeam-directing mirror 506, and is focussed by lens A on the nonlinearcrystal 502. The beam is then reflected at the reflective mirror 504,passes through the crystal 502 a second time, and re-traces its beampath through the lens A. The second harmonic portion of the beam is thenreflected by the 45° beam-directing mirror 506. The exit beam is now atan angle θ₁ from a rotated central position 501, which is at two timesthe exact angle of the beam-directing mirror 506, and thus is about 90°with respect to the central position 500.

[0215] The advantage of the structure and method shown in FIG. 58 isthat the nonlinear crystal 502 is used twice by the fundamental beam.This method can almost double the conversion for the case of lowconversion efficiency, in the case where the fundamental beam intensityis not significantly depleted in its first passage (e.g., when thewavelength conversion efficiency is less than about 30-40%).

[0216] In the present embodiment, the residual laser photons of theinitial laser beam B1 will not be used and may be filtered out by adielectric coated mirror which has bandpass characteristics of hightransmission at the second harmonic wavelengths of about 395-430 nm andblocking at the fundamental wavelengths of about 790-860 nm.Alternatively, the fundamental and the second harmonic waves can beseparated spatially using dispersive optical elements, such as a highoptical index prism or an optical grating. The filtering optics (notshown) can be placed in the beam path after the beam emerges from thefirst wavelength converter 108.

[0217] As noted above, after emerging from the first wavelengthconverter 108 (see FIG. 1), the second laser beam B2 has a wavelength inthe range of about 395-430 nm. To attain the preferred operating laserwavelengths of 198-215 nm, the laser beam is directed into a secondwavelength converter 110.

[0218] The optical arrangement of the second wavelength converter 110 isalmost identical to that of the first wavelength converter 108, which isillustrated in FIGS. 5A and 5B. The main difference between the twoconverters 108, 110 is in the optical crystal 502. For wavelengthconversion from about 395-430 nm to about 198-215 nm, the preferrednonlinear optical crystal is again beta-Ba₂BO₄ (beta barium borate). Theoperating conditions are different in that the phase matching angles areat or close to 90° for type I phase matching. The optical faces of thecrystal 502 are to be coated for maximum transmission at the frontsurface for the fundamental wave where the laser beam enters the crystal502, and for maximum transmission at the second harmonic wave on theexit face. The optical characteristics of beta-Ba₂BO₄ crystals impose alower limit of the converted wave at about 200 nm for appreciableconversion efficiency.

[0219] The third laser beam B3 emerging from the second wavelengthconverter 110 has a residual wavelength of about 390-430 nm. A wavefilter means consisting of dispersive prism or optical gratings can beused to spatially separate the 200 nm wavelengths from the 400 nmwavelength contents. The wave filter (not shown) can be placed anywherein the laser beam path after the second wavelength converter.

[0220] The two-step wavelength conversion process described above canalso be consolidated such that the fundamental wavelength can beconverted into its fourth harmonic with a single optical arrangement. Asillustrated in FIGS. 5A and 58, first and second nonlinear opticalcrystals 502, 503 (shown in dotted outline) are placed in closeproximity and are at the beam waist of the mildly focussed incidentlaser beam. The first crystal 502 is cut and oriented for phase matchconditions to generate the second harmonic wave. The first crystal 502is used to convert the fundamental wave into its second harmonic wave,and has a function as described above for the single-crystal embodimentof the first wavelength converter 108. For this purpose, in theconfiguration shown in FIG. 5A, the first crystal 502 is placed in frontof the second crystal 503, facing the incident laser beam B1 emergingfrom the scanner amplifier. The portion of the laser beam B1 convertedinto the second harmonic wavelength of about 390-430 nm after passingthrough the first crystal 502 is then incident upon the second nonlinearcrystal 503. The second crystal 503 is cut and oriented for phasematching as described above for the second wavelength converter 110,resulting in another step of second harmonic conversion, now using the390-430 nm beam from the first crystal 502 as the fundamental wave.

[0221] In the configuration shown in FIG. 5B, the second crystal 503 isplaced in front of the first crystal 502. However, in FIG. 5B, theincident laser beam B1 passes through the second crystal 503 withpractically no conversion, since the crystal is oriented for phasematching for the 390-430 nm laser pulses emerging from the first crystal502 after the pulses reflect off of the reflective mirror 504. Thus, theportion of the laser beam B1 converted into the second harmonicwavelength of about 390-430 nm after passing through the first crystal502 is reflected and then incident upon the second nonlinear crystal503. The second crystal 503 is cut and oriented for phase matching asdescribed above for the second wavelength converter 110, resulting inanother step of second harmonic conversion, now using the 390-430 nmbeam from the first crystal 502 as the fundamental wave.

[0222] Other modifications are necessary for optimal operation of such aone-step wave converter. For the optical arrangement shown in FIG. 5A,lens B is properly coated for maximum transmission (anti-reflection) atthe 200 nm range. The material for the lens is preferably UV quartz forgood optical transmission. In FIG. 5B, the modification is that thecoating characteristics of the dielectric mirror 506 be highlyreflective at about 198-215 nm at a 45° incident angle. Theseimprovements for optical transmission for lens B in FIGURE SA also applyto lens A in FIG. 5B.

[0223] In the foregoing discussion, a laser fundamental wavelength rangeof about 790-860 nm is illustrated for a Ti-doped Al₂O₃ laser. However,a Ti:Al₂O₃ laser has an operating range of about 680 nm to about 1200nm. Therefore, the wavelength conversion apparatus and method describedabove can be applied to generate a slightly extended output wavelengthrange of about 396-600 nm after the first conversion, and about 198-300nm after the second conversion, without loss of generality (the lowerlimits are about 396 nm and 198 nm, respectively, rather than about 340nm and 170 nm, because of limitations of the nonlinear conversioncrystal 502).

[0224] In an alternative embodiment, the wavelength conversion apparatusand method may include sum frequency generation with two different laserwavelengths. In this case, the first wavelength converter 108 isstructurally as described above. If the fundamental wavelength isselected to be about 790-900 nm, those wavelengths can be used to mixwith the second harmonic wave of about 395-450 nm. However, thenonlinear optical crystal 502 in the second wavelength converter 110 hasto be cut and oriented for phase matching conditions for the fundamentaland the second harmonic waves in order to generate a laser wavelength ofabout 263-300 nm. If the fundamental laser wavelength from the laserunit 100 is in the range of about 790-900 nm, the laser wavelength atthe output of the first wavelength converter 108 is modified by the wavemixing action of the second wavelength converter 110 to about 263-300nm.

[0225] Operation of the Inventive Apparatus

[0226] In order to improve the ease of use of the present invention, andto ensure proper alignment of the surgical laser beam S with respect toa target cornea, the present invention includes a guide beam unit 132(see FIG. 1). The guide beam unit 132 is illustrated in greater detailin FIG. 8.

[0227] The guide beam unit 132 includes a low-power laser 800 with anoutput of preferably less than 1 milliwatt at initial output andpreferably attenuated to the microwatt level for safe usage for directviewing. The laser 800 in the guide beam unit 132 may be, for example, aHeNe laser or a semiconductor diode laser. The laser 800 generates aguide beam 801 which is conditioned optically so that it can be used asa indicator of the location of the surgical laser beam S. Additionally,the guide beam 801 can be used as an element for the alignment of theeye in preparation for surgical procedures.

[0228] After emerging from the laser 800, the guide beam 801 thediameter of the guide beam 801 is expanded through a telescopic beammagnifier consisting of divergent lens 802 and a convergent lens 804 toabout 10 mm. The collimated beam is then compressed by a weaklyfocussing lens 806, with a focal length of over 500 mm, at a distance ofapproximately the location of the patient's cornea. An axicon first andsecond prism pair 808, 810 are aligned with the expanded beam such thata divergent ring image with uniform intensity is produced after thefirst prism 808. The second prism 810 intercept the divergent ring anddiffracts it to form a ring image 812 without divergence. The diameterof the ring image is controlled by the separation between the prism pair808, 810: the farther they are separated, the larger is the ring. Theposition of the second prism 810 can be adjusted by a manual ormotorized drive. The guide beam 801 emerges from the guide beam unit 132and is denoted as beam G in FIG. 1.

[0229] The ring-shaped guide beam G from the guide beam laser unit 132is aimed at a partially transmissive mirror 134 arranged so that thereflected guide beam G is coaxial with the un-deflected surgical laserbeam S. In operation, a surgeon would move the patient's head and targeteye until the guide beam G is roughly centered over the patient'scornea. The surgeon will then adjust the diameter of the ring imageprojected on to the patient's pupil, such that the ring diameter is onlyslightly less than the patient's pupil size. At that point, the patientwill see the guide beam G, but not necessarily at a centered position.

[0230] The patient is preferably situated in a relaxed position (e.g.,supine), but with his or her head fixed within a cradle or fixture.Either the cradle or the entire operating table or chair is configuredto be adjustable in fine increments about an X-Y plane perpendicular tothe surgical laser beam S. The patient would then make fine adjustmentsof his or her own eye position with respect to the guide beam G bymoving an actuator-control mechanism (e.g., a joy stick) for the cradleor operating table or chair, until the patient determines that the guidebeam G appears to be at its brightest. At the completion of thepatient's adjustment, the patient's eye is aligned with the patient'svisual axis, coinciding with the guide beam G.

[0231] An advantage of the ring-shaped guide beam G of the presentinvention over a solid beam is that the light fall-off, or decrease inbrightness, is greater for the ring-shaped beam than for the solid beamwhen either beam is not aligned with the patient's visual axis. Thus,the ring-shaped beam provides greater visual cues to the patient whenthe beam is off-axis.

[0232] After the eye of the patient has been aligned using the guidebeam G, the surgeon may place a suction ring 400 over the patient's eyeto immobilize it. The eye tracking system 130 is then activated tocompensate for any subsequent motion of the eye. With the eyeimmobilized, the surgeon may then commence ablative surgery using theinventive laser system.

[0233] To determine the location of each area to ablate, and the depthof ablation required, an automatic feed-back control system may be usedwith the inventive system. Such a control system preferably includes acorneal profiler 136 which provides information to the computer controlunit 114 sufficient to determine the necessary intensity and XY-scanningcoordinates for the surgical laser beam S, and to otherwise control thedelivery of pulses of laser energy to the cornea, in order to achieve adesired corneal surface profile. A suitable corneal profiler 136 is anydevice that measures the shape or an optical property of the eye so asto provide such information.

[0234] As an alternative to a single profile measurement and ablation ofthe cornea based on indicated parameters, a desired corneal surfaceprofile may be obtained through ablation by a successive approximationtechnique. In this technique, a measuring device is used to determinethe change desired to be made in the profile of the corneal surface.Pulses of laser energy are delivered to the surface so as to bring aboutslightly less than the desired degree of alteration. The measuringdevice is then used again to determine the correction now needed toreach the desired profile. Further pulses of laser energy are providedaccordingly to produce slightly less than the total calculatedcorrection. This process is repeated until the ablated surface acquiresthe desired profile to a suitable degree of accuracy.

[0235] Measurement devices suitable for the corneal profiler 136 arekeratometers, which are known and commercially available. Examples ofsuch devices are the “Photokeratoscope” manufacture by the Sun ContactLens Company of Kyoto, Japan, and the “Corneascope” manufactured byInternational Diagnostic Instruments, Ltd., Broken Arrow, Okla., USA.(See also S. D. Klyce, “Computer Assisted Corneal Topography”, Invest.Ophthalmol. Vis. Sci. 25:1426-1435, 1984 for a comparison of theseinstruments and a method of using the “Photokeratoscope”). These deviceswork by imaging patterns, usually concentric rings, on the cornealsurface. Preferably, the keratometer used as the corneal profiler 136 inthe present method is modified slightly to increase the number of linesimaged on the central portion of the cornea, thus increasing themeasurement resolution of the curvature of the central portion.

[0236] In the preferred embodiment, the corneal profiler 136 receives areflected image of a target cornea by means of a mirror 138, which ismovable between an out-of-line position A and an in-line position B.When the mirror 138 is in position B, an initial profile of the corneacan be determined by the corneal profiler 136. The output of the cornealprofiler 136 is coupled to the computer control unit 114, and displayedto a surgeon. In response to any resulting input from the surgeon, suchas the desired final shape of the corneal surface, the computer controlunit 114 determines the necessary settings and parameters, includingpulse intensity, beam diameter, and target locations on the cornea, forthe inventive laser system to create the desired ablation profile. Themirror 138 is then moved to position A, and the surgery commenced.

[0237] If the successive approximation technique described above isused, the mirror 138 is periodically moved back into position B, thecorneal profile is re-measured, the computer control unit 114 resets thelaser system, the mirror 138 is retracted to position A, and the surgeryre-commenced.

[0238] In determining the necessary settings and parameters, includingpulse intensity, beam diameter, and target locations on the cornea, forthe inventive laser system to create the desired ablation profile, thecomputer control unit 114 essentially prepares a three-dimensionalcontour map of the difference between (1) the cornea profile as measuredand (2) the desired final shape of the cornea. Each point in thiscontour map may be described in terms of rectangular or polarcoordinates, in known fashion. Starting with a selected pulse etchprofile (i.e., a selected beam intensity and intensity profile), theetch depth of each pulse will be known (such information can bedetermined in advance by calibrating sets of profiles on cornealtissue). With a preselected etch depth, the contour map can be dividedinto a plurality of etch levels (for example, of the type shown in FIG.7B). Then, using a selected initial laser pulse diameter, each level ofthe contour map may be characterized in terms of the X-Y coordinates ofa first level pattern of etch profiles of that diameter.

[0239] For example, if the area to be ablated has a maximum diameter of8 mm, and the laser pulse diameter is about 0.1 mm, then a grid of 80×80pulses will cover the entire maximum diameter. Arbitrarily selecting asingle origin for such a grid means that each point on a level of thecontour map can be defined in terms of X-Y coordinates. As the levelsbecome smaller in size, the laser pulse diameter may be reducedaccordingly, but the principal of mapping each level of the contour mapto a grid of X-Y coordinates remains the same.

[0240] Structure of the Scanner-Amplifier Laser Unit

[0241] This part of the disclosure is directed to a laser amplifiersystem utilizing a pair of scanning mirrors driven in tandem by piezoactuators. A control system is provided to direct a low-power laser beamwhile the beam is trapped and circulates between the pair of scanningmirrors. Each bounce of the laser beam between the mirrors discretelyincreases the power of the beam and changes the angle of exit of thebeam from the amplifier, providing for precise angular beam exit controlin two dimensions.

[0242] In the preferred embodiment, a laser scanner-amplifier system 8with Ti-doped sapphire Al₂O₃ is used as the laser medium. However, thelaser medium can be other tunable solid state laser materials, such asalexandrite, emerald, Cr:LiCaF, Cr:LiSrF, Cr:forsterite, color centerlasers, or rare earth ion laser media, such as Nd, Pr, Er, Tm, Ho, orother transition metal ions such as Co, Ni in various solid statecrystal hosts, including oxides or fluorides.

[0243] A laser pulse train from a mode-locked Ti-doped Al₂O₃ laser 10 inFIG. 11 is to be used as a seeder to the amplifier scanner system. Thelaser pulse frequency of the mode-locked laser, as is well known in theart, can be controlled by the round trip time of the laser pulse insidethe mode-locked laser, and is at twice the driver frequency of theelectrical signal applied to the mode-locked crystal. The frequency ischosen such that time period between adjacent pulses bears a preferredrelationship with the arrangements of the optical elements inside thescanner-amplifier system. In the case of Ti-doped Al₂O₃, a continuouswave laser 12 such as, but not limited to, an argon gas laser operatingat 514.5 nm or a frequency-doubled YAG or YLF laser at 532 nm and 527 nmrespectively, can be used as the pump source. The pump laser beam 13 isfocused into the mode-locked laser medium with a convergent lens 14. Thearrangement of a laser-pumped mode-locked laser is well known in the artand a commercial model is available from Spectra-Physics, Mountain View,Calif.

[0244] The mode-locked laser beam 15 passes through a set of beamconditioning optics 16, 18. In FIG. 11, the beam cross-section isexpanded by a negative (concave) lens 16 and a positive (convex) lens 18with their focuses coinciding to form an expansion telescope. Theexpansion ratio can vary from 2 to 10 by choosing the appropriate focallengths of the optical elements 16 and 18, and is determined by themode-matching requirement between the seed beam 15 and the spatial modeof the amplifier cavity. By centering the lenses along the laser beam,minimum beam distortion and good beam collimation can be achieved as theseed beam 15 exits the optical element 18.

[0245] The seed beam is directed by high reflective mirrors 20 and 22into the amplifier cavity. The beam first enters the cavity through adielectric coated mirror 24 which has the optical characteristics that api-polarized laser beam with the electric field vector horizontal to theplane of incidence has over 96% transmission, and a pi-polarized laserbeam with the electric field vector vertical to the plane of incidencehas over 99% reflectability. Such thin-film polarizer elements aresupplied by Burleigh NorthWest, Fishers, N.Y. The scanner-amplifiercavity 8 is confined between the scanner mirrors 26 and 28, both ofwhich are highly reflective mirrors. The scanner mirrors 26, 28 are eachmounted on a gimbal mount 29 with 90° tilts in both the horizontal andthe vertical (X-Y) directions. The design of the gimbal mount can beillustrated as a mirror mount model number MM-1 manufactured andsupplied by the Newport Corporation, Fountain Valley, Calif., withappropriate modifications to shorten the pivot point distance andincrease the spring force. The X-Y tilts are achieved by piezoelectricactuators 31 with a material such as PZT which can have a linear travelof 40 microns of full scan range at about 1000 Hz, and at higherfrequencies with smaller travel range. Such piezo actuators are suppliedby a number of suppliers, including Burleigh Instruments, Fishers, N.Y.The scan mirrors 26 and 28 are driven in the same direction at the sameangular degree either independently or in tandem in both the X and Ydirections.

[0246] The operating characteristics of the piezo actuators may havesmall variations. The overall scan angles of the laser beam as emergedfrom the scanner-amplifier is to be calibrated against the voltageapplied to the piezo actuators 31, taking into the account the smallamount of hysteresis from the piezoelectric effect.

[0247] A pair of concave lenses 30 and 32 are included inside thescanner-amplifier cavity. The focal lengths of the lenses 30 and 32 aresuch that the focal length of lens 30 is chosen to be as large aspossible, yet the size of scanner-amplifier is to be practical andconvenient for use, and the focal length of lens 32 will be as short aspossible, yet not so short as to cause optical break-down at its focalpoint. The relative locations of the lenses 30, 32 and end mirrors 26and 28 are such that the mirrors 26 and 28 are to be at the focal pointof the lenses 30 and 32, respectively, and the separation between thelenses is to be the sum of their focal lengths. Anotherdielectric-coated mirror 34, which has similar characteristics as mirror24, is used as a turning mirror and also as an exit mirror where thelaser beam 15, intensity amplified and scan-angle amplified, emergesfrom the scanner-amplifier unit 8.

[0248] Other control elements inside the cavity include a Pockels cell36 which consists of LiNbO₃ or other electro-optical crystal such asKDP. Pockels cells are commercially available from several sources, onesuch source is Medox Electrooptics, of Ann Arbor, Mich. With theapplication of electric voltage across the electro-optical crystal, ahalf-wave retardation in the electric field vector of the laser beam canbe generated, which turns the linear polarization of a laser beamtraversing the crystal, from a horizontal polarization to vertical, andvice versa. A half-wave retardation plate 38, placed next to the Pockelscell 36, is for adjusting the polarization of the beam before it reachesthe mirror 34, so that the beam will either stay inside the cavity orexit the cavity at mirror 34.

[0249] A thin etalon 40 with partial reflective coating on both faces atthe laser wavelength is for controlling the gain bandwidth of the seedbeam 13. By choosing the appropriate finesse of the etalon, thewavelength width of the laser beam is reduced accordingly, compared tothe seed beam bandwidth. The pulse duration is is lengthened due to thereduced spectral content in the laser pulse.

[0250] Another method of expanding the pulse duration can be achieved bystretching the pulse spatially with an optical grating, before the pulseis injected into the beam path at location 21 shown in drawing FIG. 11.For shorter pulses, a commercial pulse compressor unit, consistingbasically of a single-mode fiber and grating pair, can be placed atlocation 21 instead of just the optical grating. Such a unit ismanufactured by Spectra-Physics Lasers, Mountain View, Calif.

[0251] Hence, the output laser pulse can be varied from a minimum whichis that of the seed pulse, which is about 1 picosecond in the case ofTi:Al₂O₃ as the laser medium in the mode-locked laser, to as much asseveral hundred picoseconds.

[0252] Referring now to FIG. 11A, in a second embodiment, a laser gainmedium 42 is located near the scanner mirror 26. A cavity aperture 44which has a fixed or adjustable iris with two translational degrees offreedom for proper centering with the fundamental laser mode is locatedinside the cavity. The laser media is optically pumped by a laser source48 which will be described hereinafter in more detail. The secondembodiment provides enhancement of the laser beam intensity inside thescanner cavity, such that the beam intensity increases by extractingenergy stored in the gain medium 42.

[0253] Operation of the Preferred Scanner-Amplifier Laser Unit

[0254] For the purposes of illustration, an angle is being scanned inthe horizontal plane (the X-plane). A scan voltage is applied to bothpiezo actuators 31 for positioning the gimbal mirror mounts for scanmirrors 26 and 28 in the same direction to the same degree; as anexample, both pushing the mirrors forward as shown in FIG. 11. Ahalf-wave voltage electrical wave form signal is applied to the Pockelscell, as illustrated in FIG. 12B. The time sequence from 2(i) to 2(vi)marks the time development of the optical retardation of the Pockelscell 36. A voltage is to start at time 2(ii), and the opticalretardation reaches half-wave at time 2(iii). The voltage is turned offat time 2(iv), and zero retardation is reached at time 2(v). The timeduration between 2(ii) and 2(iii) is referred to as the rise time of thePockels cell for a half-wave retardation. The duration between 2(iv) and2(v) is the fall time for the same.

[0255] Since the seed laser pulse is in the picosecond range, thespatial extent of the laser energy is localized in the range ofmillimeters. The cavity distance between scan mirrors 26 and 28 is, forpractical purpose, in the range of tens of centimeters to tens ofmeters. Therefore, for all practical purpose, the laser pulse can beconsidered localized and is represented by markers 2(i) to 2(vi) as ittravels through the scanner-amplifier cavity. The seed laser beam, attime 2(i) travels towards the scanner-amplifier cavity, and entersthrough the thin film polarizer mirror 24. As illustrated in FIG. 12A,the beam 15 has a linear polarization with the electric field vector inthe horizontal direction, as indicated by the arrow. The beam passesthrough the lens 30, and is focussed at a point before lens 32 whichcollimates the beam due to the confocal arrangement of the lenses 30 and32. The Pockels cell (PC) voltage is at the zero level, and thepolarization of the seed beam is not changed. The Pockels cell voltagethen turns on at time 2(ii), right after the laser pulse exits the PCcrystal. The polarization changes by 90° after passing through thehalf-wave plate 38, and is now vertical, as indicated by a small circleon the beam path. The beam is then reflected by the thin film polarizermirror 34 directing the beam towards the scan mirror 28.

[0256] In FIG. 13, the beam path and the angle of incidence at mirrors26 and 28 are illustrated. Assume that a voltage V₁ is applied to thepiezo actuator 31, which induces a scan angle of θ₁ from its zero degreeincidence, at which the mirror is at the normal incidence with theincoming seed beam. The reflected beam is at an angle, 2 times θ₁ fromthe incoming beam. Referring again to drawing FIG. 12A, the beam isreflected at mirror 28. The vertical polarization of the beam changes by90° after passing through the half-wave plate 38. The PC voltage reacheshalf-wave retardation at 2(iii) (see FIG. 12B) before the laser pulsereaches the PC. On passing the PC, the polarization is rotated 90° andis now vertical. The lens 30 re-collimates the laser beam 15 and thethin film polarizer 24 is now at high reflection with respect to thevertically polarized beam. The beam 15 then travels towards the lasergain medium 42 and the cavity aperture 44.

[0257] Assuming that a voltage V₂ is applied to the actuator 31 in themirror gimbal mount 29 for the scan mirror 26, an angle rotation of θ₂from the normal incidence results in the X-plane, where the normalincidence is defined as the scan mirror angular position for bothmirrors 26 and 28 at which the seed laser beam 2(i) will retrace itsbeam path after reflection from both these mirrors. The reflected beamis, therefore, at a larger angle than the incident angle beforeimpinging on the mirror 26, by an angle 2 time θ₂, as shown in FIG. 13.

[0258] For ease of explanation, the following discussion is directed toejecting the laser beam after only one reflection from each of themirrors 26 and 28; however it should be understood that it iscontemplated that a plurality of reflections occur from each mirrorwithin the device prior to the beam exiting therefrom. By so choosing,the PC voltage turn-off starts after the beam emerges from the PC attime 2(iv), and the retardation is zero at 2(v) before the beam reachesthe PC on its return trip from the scan mirror 26. The verticalpolarization remains vertical after passing the PC, and is rotated tohorizontal after the half-wave plate 38. The thin film polarized mirroris now transmissive for the laser beam, and the laser beam emerges fromthe amplifier-scanner of the invention with a scan angle resulting fromthe sum of the effects of the scan angles θ₁ and θ₂ from the scanmirrors 28 and 26 respectively.

[0259] It should be understood that invention makes use of the scanmirrors 26 and 28 repeatedly for one or more round trips of the beaminside the cavity to amplify and precisely direct the beam angle beforeexiting mirror 34.

[0260] In our preferred embodiment, the PC voltage turn-off, at times2(iv)-2(v), is to be applied at the last leg after one or more roundtrips between the two scan mirrors 26 and 28. In the case where thevoltage turn-off is postponed, as in the illustration in FIG. 12A, thepolarization of the reflected beam from mirror 26 is rotated tohorizontal after the PC, which is still at its half-wave voltage, andback to be vertical again after the half-wave plate 38. Therefore, themirror 34 is highly reflective. The beam is trapped inside the cavity,and the beam angle increases with each reflection with either of thescan mirrors.

[0261] Further, in addition to changing the beam angle, the opticalarrangement enhances the overall scan angle of the beam with a powermultiplying enhancement factor.

[0262] If the focal length of the lens 30 is longer than that of lens32, by a factor M, then:

M=f₍₃₀₎/f₍₃₂₎

[0263] where f₍₃₀₎, f₍₃₂₎ are the focal lengths of the lenses 30 and 32,respectively. The angle of incidence on mirror 28 is θ₁, and the angleof incidence on mirror 26 is: θ₁/M+θ₂.

[0264] Notice the angle reduction of θ₁ due to the difference in thefocal length of the lenses.

[0265] On passing through the lenses system from 30 to 32, the reverse,i.e., a magnification of the effective angle, occurs. The incident angleon mirror 28 is now: (θ₁/M+θ₂)×M+θ₁.

[0266] In the illustration in FIG. 12A, in which the laser beam is toexit the cavity after one reflection from mirrors 26 and 28, the outputbeam would have a scan angle of: 2×(θ₁+M×θ₂).

[0267] Notice that the scan angle due to mirror 26, θ₂, is magnified bya factor M.

[0268] If a total of N reflections are allowed to occur for each of thetwo scan mirrors, the final scan angle of the exit beam is:2N×(θ₁+M×θ₂).

[0269] Since each reflection or transmission on an optical surfacecauses a certain amount of intensity loss and optical distortion in thelaser beam, ideally the intended scan angle will be achieved with thesmallest number of optical surface contacts. If the scan mirrors haveidentical gimbal mounts 29 and piezo actuators 31, the mirrors can bescanned in tandem, and θ₁ and θ₂ will be substantially equal. Theoptical loss due to scattering from all the optical elements inside thecavity is reduced by the factor: (M+1)/2

[0270] For M=3, and 10 round trips inside the cavity, the scan angle isamplified by 20 times more than the amplification of the scan anglesfrom two like but uncoupled piezo mirrors.

[0271] It is also clear that all the foregoing discussion about scanningin the horizontal direction is also applicable to the vertical direction(a Y-scan), by applying the scan voltage to the piezo actuator whichcontrols the vertical tilt of the scan mirror. By applying theappropriate voltages to the actuators controlling the horizontal and thevertical scan directions, the laser beam can be directed to anypredetermined location in the two dimensional angular space.

[0272] The pump source 48 of the Ti:Al₂O₃ in the amplifier cavity inFIG. 11 consists of two major components, namely, a Nd-doped YAG or YLFlaser which is continuously pumped by arc lamps such as Kr or Ar gaslamp, which is supplied by ILC Technology, Sunnyvale, Calif., or bysemiconductor diode arrays with the emission laser wavelength to matchthe absorption band of Nd-doped YAG or YLF. Several hundred to over onethousand watts of continuous wave laser output power from Nd:YAG isattainable with multiple lamp-pumped laser heads inside a laser cavity.Such lasers are supplied by Lasermetric, Orlando, Fla., and a number ofother industrial YAG laser suppliers.

[0273] In a preferred embodiment, the Ti ion has an absorption bandcentered at about 520 nm, with a full width at half maximum of about 100nm. The second harmonic wavelengths of the Nd-doped YAG and YLF arecentered around 532 nm and 527 nm, respectively, and both are suitableas a pump source.

[0274] In the second harmonic generation (SHG) process, one of limitingfactors in the conversion efficiency and the power stability is thetemperature gradient induced by absorption of the laser at itsfundamental and second harmonic frequency. Choosing a second harmoniccrystal with good thermal conductivity, and cooling the crystal byliquid flow or by contact cooling, are among the common methods toextend the upper limit of the input fundamental laser power for the SHGcrystal.

[0275] Referring now to drawing FIGS. 14A, 14B and 14C, the output laserbeam 55 of a high power, acoustic-optical switched, Nd-doped YAG or YLFlaser beam source 56 is directed to a series of partially reflectingbeam splitters 57, which are coated with dielectric so that, at the 45°incidence, they all have high transmission for the second harmonicwavelength, and each succeeding splitter is highly reflective at thefundamental wavelength of the laser source 56, so that the laser beampower is distributed equally among each branch when they are directedtowards the SHG crystals 60. The crystal 60 is chosen for high nonlinearcoefficient, good acceptance angle, and high tolerance to a temperaturegradient. KTP is among the top choices as a SHG crystal for conversionat 1.04 to 1.06 microns.

[0276] In a preferred embodiment, 20-60 watts of average power will beachieved in the beams 1-5 of FIG. 14A. To further increase theconversion efficiency, a convex lens 58 can be inserted between eachsplitter 57 and each SHG crystal 60, such that the crystal is at thefocal distance f₍₅₈₎ from the lens, where the beam cross-section is thesmallest and the laser power density is the highest. The focal length ofthe lens is chosen to optimize for the acceptance angle of the SHGcrystal. A spherical concave mirror 62 that is highly reflective at boththe fundamental and the second harmonic wavelength is placed at theradius of curvature of the mirror 62, R₍₆₂₎, from the first surface ofthe crystal, where the laser beam enters the crystal. This opticalarrangement allows for the return beams of both the fundamental and thesecond harmonic to retrace the beam path of their first passage in thecrystal, and ensure a good beam overlapping in the crystal even thoughthere may be walk-off between the beams after their first pass.

[0277] To illustrate our embodiment, we combine five beams at the secondharmonic wavelength with a novel spatial combiner 64. As shown in FIG.14B, the combiner 64 is a six-face optical element which has four sides63 a, 63 b, 63 c and 63 d, each of which form a 45° angle with the baseface 67, and a top face 65 which is parallel to its bottom face 67. Theside faces are coated for high reflectivity at 45° at the secondharmonic wavelength, and the top and bottom faces are coated with ananti-reflection coating at the second harmonic wavelength. As shown inFIG. 14C, by using beam steering optics, the five beams from FIG. 14Acan be reflected off the side faces of the combiner 64, and one beam(beam 2) in FIG. 14C can transmit through the parallel faces. The beamsare adjusted such that they re-collimated and are parallel with eachother. A convex lens 66 is centered symmetrically in the beam path, andfocuses the five beams into a common focal point. This optical element66 can be a replacement of or an equivalent to the element 46 of FIG.11.

[0278] It also follows from the present invention that additional beamscan be combined with a spatial combiner with additional facets on thecombiner. As an example, a hexagon, instead of a square top, can combineup to 7 beams.

[0279] In another embodiment, the facets can be formed on more than onelayer, such as 4 facets on the top tier and 6 facets on the second tier.

[0280] In all end pumping configurations, the pump beam is absorbed bythe laser active ions in the crystal host. The energy distribution inthe laser medium is a negative exponential function, with a maximum atthe entrant face. For efficient cooling, and to minimize the distortionof the laser beam, the laser medium in the invention is to be in acylindrical laser rod form. A conventional laser rod is mounted with theend faces outside of the contact with the coolant. In FIG. 15A, thepreferred embodiment consists of a Ti:Al₂O₃ laser rod with a recessedcollar 50. A thin wall tube made of undoped sapphire 52 is to fit at theend sections of the laser rod. The tube piece is glued to the laser rod,and the whole has a cylindrical shape as shown in FIG. 15B. Thiscylindrical piece is then mounted to a liquid cooled envelope similar tothe ones used in an arc lamp pumped laser. A water flow channel aroundthe laser medium and the extension is shown in FIG. 15B, in which thewater inlets and outlets are shown schematically. O-rings 54 areretained in such a manner that the coolant is sealed from coming intocontact with the flat laser surfaces of the laser rods. The tubeextension allows the whole laser medium to be in contact with the liquidcoolant. Using the same material in the extension tube also minimizesstress as a result of a difference in thermal expansion coefficient,with temperature variation in the whole assembly.

[0281] In another embodiment, an additional pump source can be appliedthrough mirror 24 collinear with laser path from pump source 48, suchthat the laser media is pumped from both ends.

[0282] In another embodiment, additional laser media is to be includedin front of the scan mirror 28, and a pump configuration identical tooptical elements 46 and 48, pumping one end of the laser medium, orpumping from both ends of the laser medium, is to be applied to thelaser medium near mirror 28.

[0283] Multi-kilohertz laser operation is achieved with the followingmethod. A synchronized electrical wave form is tapped from the modelocker driver 66. According to the desired repetition rate, thesynchronized signal can be divided electrically by a timer dividercircuit 68, as shown diagrammatically in FIG. 16. The resultantfrequency output of the timer-divider determines the laser frequency ofthe scanner-amplifier system. The output electrical signal of thedivider box is then time-delayed through delay generators 70 and 74,commercially available from Stanford Research Systems, Sunnyvale, Calif.One of the delayed signals 71 is fed into a Q-switched driver 72 in thepump laser 48, and a second time-delayed signal 75 is fed into thePockels cell driver 76.

[0284] The timing of the electrical signals and the laser events areillustrated in FIG. 17. In the top trace 7(a) of FIG. 17,multi-megahertz (30-200 MHz) mode locked laser pulses are represented byequally spaced laser spikes at time intervals equal to twice the modelocker driver frequency. After the timer-divider circuit, electricalsignals at multi-kilohertz (1,000-50,000 Hz) is generated at the outputof the timer-divider box, as represented by the trace 7(b). At a timedelay T₁, the Q-switch driver for the pump laser is turned on, in trace79(c), generating a short pulse of the second harmonic laser pump pulseat a time delay T_(r), corresponding to the build up of the pump pulse,a characteristic of the pump configuration, and the gain factor at thepump laser medium. The second harmonic pump pulse is absorbed in theTi:Al₂O₃ laser medium, in trace 7(d). The Pockels cell is switched on ata time delay T₂ relative to a synchronized timer-divider signal, whichis the pulse after the one that triggers the Q-switch driver. The timedelay T₂ is determined by the actual location of the seed laser pulsefrom the mode locked laser, as aforementioned along with the discussionof FIG. 12A. The delay time T₁ is to be adjusted so that the peak of thepopulation inversion is to occur when the Pockels cell crystal reachesthe half-wave retardation point of 2(iii) as shown in FIG. 12B.

[0285] Applicable Surgical Procedures

[0286] The laser surgical system of the present invention can performnumerous types of surgical procedures on the cornea. Among otherprocedures, two types of laser tissue interaction are particularlysuited for the inventive system:

[0287] (1) The inventive system can easily create straight line andcurved-line incisions, of any predetermined length and depth, at anylocation determined by a surgeon.

[0288] As illustrated in FIG. 9A, multiple radial cuts 902, equal orpartially equal in incision length and with an angular separationbetween cuts, can be made on the cornea with the present surgicalsystem. An incision can be made by directing the surgical laser beam Sto a predetermined location at the cornea, and removing the desiredamount of tissue by controlling the laser beam energy dosage. Thepresent invention provides options for making an incision with either awide incision width by using a larger beam spot size on the corneasurface, or a fine incision by using a more focussed beam spot. With thepresent invention, the depth of each cut can be varied over the lengthof the cut.

[0289] In FIG. 9B, a side view of a cross-section of the cornea shows ashallower cut depth 904 near the central region of the cornea and adeeper cut depth 905 near the outer edge of the cornea. Such a procedureprovides more uniform stretching of the cornea from the central to theedge regions, and increases visual acuity post-operatively.

[0290] The invention can also easily generate transverse cuts(“T-cuts”), as shown in FIG. 9C. By directing the surgical laser beam Sto make a pair of opposing transverse incisions 906 along an axis 908relative to the center of the eye, the refractive power of eye isdecreased along the axis. The exact length d and the location of theincision can vary according to the amount of desired correction, inknown fashion.

[0291] The inventive system can also be used for procedures in corneatransplants. A circumcision of the cornea in any predetermined shape(e.g., circular, elliptical, hexagonal, etc.) can be performed on thedonor eye and the recipient's eye. In both cases, the computer controlunit 114 calculates the beam location based on the particular shaperequired, and the amount of laser energy needed to cut through thecornea.

[0292] In general, incisions in the cornea can be made at effectivelocations for performing radial keratotomies or making T-cuts, tocorrect myopia, hyperopia, or astigmatism.

[0293] (2) The second important type of laser-tissue interactionprovided by the inventive system is area ablation, which permits directsculpting of the corneal surface.

[0294] As illustrated in FIG. 10A, a local scar or infected tissue canbe removed with the present invention. The defective tissue is removedto a desired depth d over a predetermined area on the cornea. A donorcornea cap can be cut and ablated (“sculpted”) to the desired dimensionand thickness using the invention. The cap piece is then transferred tothe bared stroma bed and attached by suture, glue, or other appropriatemeans, in known fashion.

[0295] Again in FIG. 10A, an alternative method is shown for performinga cornea transplant. The invention can be used to ablate the cornea mostof the way or all of the way through from the epithelium to theendothelium of the cornea. Then a donor cornea 1001 is cut to matchingdimensions, and attached to the open ablated area by sutures or otherknown methods.

[0296] For myopia correction, as illustrated in FIG. 10B, the curvatureof the cornea can be reduced by selectively ablating the cornea in sucha way that more tissue is removed at the center portion C of the cornea,with a decreasing amount of tissue being removed towards the periphery Pof the cornea. Prior to the laser procedure, the epithelium optionallymay be removed by mechanical means. The new desired profile of the eyemay include the Bowman's membrane and part of the stromal layer,depending on the amount of refractive correction required. As describedearlier, the computer control unit 114 provides for the sequence,location, and intensity of laser pulses to be deposited. The depositionpattern is preferably in accordance with the patterns discussed above inthe section “Method of Depositing Laser Pulses”.

[0297] For hyperopia correction, as illustrated in FIG. 10C, theobjective is to increase the curvature of the eye. Cornea tissue is tobe removed in increasing thickness from the center portion C out towardsthe periphery P of the cornea. Depending on the amount of correction inthe refractive power, the etch gradient for the removed tissue varies.As indicated in FIG. 10C, the depth of the removed tissue againdecreases near the periphery of the eye for a smooth transition. Thesize of the usable central region R varies depending on the amount ofhyperopic correction.

[0298] The invention is particularly useful for the correction ofasymmetric refractive errors. Irregular distortions may result from poormatching of a cornea from a transplant, uneven suturing, or fromimperfect refractive surgical procedures such as lamellar keratomileusisor epikeratophakia. The inventive system can direct the surgical laserbeam S to any desired location to sculpt the cornea according to apredetermined shape. The surgical laser beam thus can be applied tosmooth out an irregular profile.

[0299] Another use of the invention is to produce standard or customsculpted cornea caps in advance of need. The invention can be used on adonor cornea or a synthetic cornea substitute to ablate a desiredprofile to correct for myopia, hyperopia, or astigmatism. Such sculptedcaps can then be attached to a properly prepared cornea, in knownfashion.

[0300] Summary

[0301] A number of embodiments of the present invention have beendescribed. Nevertheless, it will be understood that variousmodifications may be made without departing from the spirit and scope ofthe invention. For example, while the invention has been described interms of rectangular coordinates, equivalent polar coordinates may beused instead. In addition, other lasing media may be used so long as theresulting wavelength, pulse duration, and pulse repetition rate iswithin the corresponding ranges set forth above. Accordingly, it is tobe understood that the invention is not to be limited by the specificillustrated embodiment, but only by the scope of the appended claims.

1. A pulsed laser apparatus for providing smooth ablation of materialfrom generated laser pulses, wherein each pulse of the laser apparatusprovides an ablation depth of about 0.2 microns or less, and has anablation energy density less than or equal to about 10 mJ/cm².
 2. Thepulsed laser apparatus of claim 1 , wherein the ablation depth of eachlaser pulse is about 0.05 microns or less.
 3. The pulsed laser apparatusof claim 1 , wherein the apparatus includes a laser emitting pulseshaving a wavelength in the range of about 198-300 nm, and a durationin-the range of about 1-5,000 picoseconds.
 4. The pulsed laser apparatusof claim 3 , wherein the lasing medium of the laser is selected from thefollowing group: Ti-doped Al₂O₃, alexandrite, emerald, Cr:LiCaF,Cr:LiSrF, Cr:forsterite, color center lasers, or rare earth ion lasermedia in a solid state crystal host.
 5. The pulsed laser apparatus ofclaim 1 , wherein the laser emits up to approximately 50,000 pulses persecond.
 6. The pulsed laser apparatus of claim 1 , further includingmeans for positioning the laser pulses within a selected area.
 7. Thepulsed laser apparatus of claim 1 , further including means fordetermining the position of the laser pulses within an area.
 8. Thepulsed laser apparatus of claim 7 , wherein the means for determiningthe position of the laser pulses within an area includes photodetectormeans for determining the two-dimensional position of an incident lightpulse.
 9. The pulsed laser apparatus of claim 8 , further includingmeans for reducing the diameter of the incident light pulse.
 10. Thepulsed laser apparatus of claim 8 , wherein the laser pulses have anundeflected position, and further including means for magnifying thedeflection of the incident light pulse.
 11. The pulsed laser apparatusof claim 1 , further including means for controlling the diameter of thelaser pulses.
 12. The pulsed laser apparatus of claim 11 , wherein themeans for controlling the diameter of the laser pulses is a motorizedzoom lens.
 13. The pulsed laser apparatus of claim 1 , further includingmeans for determining the diameter of the laser pulses.
 14. The pulsedlaser apparatus of claim 13 , wherein the means for determining thediameter of the laser pulses includes: a. means for enlarging thediameter of an incident light pulse; b. means for directing at least apart of the light energy of the laser pulses through the pulse diameterenlarging means; c. imaging means for determining the diameter of anincident light pulse from the pulse diameter enlarging means.
 15. Thepulsed laser apparatus of claim 14 , further including means fordetermining the intensity profile of the laser pulses.
 16. The pulsedlaser apparatus of claim 1 , further including means for controlling theintensity of the laser pulses.
 17. The pulsed laser apparatus of claim16 , wherein the means for controlling the intensity of the laser pulsesincludes an electro-optical filter.
 18. The pulsed laser apparatus ofclaim 17 , wherein the electro-optical filter includes a Pockels celland a polarizer.
 19. The pulsed laser apparatus of claim 1 , furtherincluding means for determining the intensity of the laser pulses. 20.The pulsed laser apparatus of claim 19 , wherein the means fordetermining the intensity of the laser pulses includes: a. photosensormeans for determining the intensity of an incident light pulse; b. meansfor directing at least a part of the light energy of the laser pulses tothe photosensor means.
 21. The pulsed laser apparatus of claim 1 ,further including means for controllably blocking the laser pulses fromthe material.
 22. The pulsed laser apparatus of claim 21 , wherein themeans for controllably blocking the laser pulses from the materialincludes a Pockels cell and a polarizer configured as a controllablelight valve.
 23. The pulsed laser apparatus of claim 3 , wherein thelaser pulses are wavelength converted to the range of about 198-300 nmby at least one wavelength converter.
 24. The pulsed laser apparatus ofclaim 23 , wherein the laser pulses are deflectable around a pivot pointfrom an undeflected position, wherein the at least one wavelengthconverter includes: a. at least one optical wavelength conversion means,for converting the fundamental wavelengths of original incident laserpulses into corresponding second harmonic wavelengths; b. a first lens,positioned at about the distance of its focal length from the pivotpoint of the laser pulses, for receiving incident laser pulses andorienting the incident angle of such laser pulses relative to a firstsurface of the at least one optical wavelength conversion means suchthat the incident angle remains constant regardless of the angle ofdeflection of the laser pulses; c. a second lens, positioned at aboutthe distance of its focal length from the at least one opticalwavelength conversion means, for receiving incident laser pulses fromthe at least one optical wavelength conversion means and for orientingsuch laser pulses such that the laser pulses are re-collimated afterpassing through the second lens; d. wavelength separation means forspatially separating the fundamental wavelength of the original incidentlaser pulses from wavelength converted laser pulses exiting from thesecond lens.
 25. The pulsed laser apparatus of claim 24 , wherein asecond optical wavelength conversion means is adjacent to a firstoptical wavelength conversion means.
 26. The pulsed laser apparatus ofclaim 24 , wherein the first lens and second lens are the same lens, andfurther including: a. reflecting means, adjacent the wavelengthconversion means, for reflecting laser pulses exiting the wavelengthconversion means back through the wavelength conversion means to thelens; and b. angled reflecting means for reflecting wavelength convertedlaser pulses exiting the lens.
 27. The pulsed laser apparatus of claim26 , wherein a second optical wavelength conversion means is adjacent toa first optical wavelength conversion means.
 28. The pulsed laserapparatus of claim 24 , wherein the optical wavelength conversion meansincludes a nonlinear optical crystal having phase matching angles sothat an incident fundamental laser wavelength within a range of about790-1200 nm is converted to its second harmonic at a wave-length in therange of about 395-600 nm.
 29. The pulsed laser apparatus of claim 28 ,wherein the nonlinear optical crystal is beta-Ba₂BO₄.
 30. The pulsedlaser apparatus of claim 24 , wherein the optical wavelength conversionmeans includes a nonlinear optical crystal having phase matching anglesso that an incident fundamental laser wavelength within a range of about395-600 nm is converted to its second harmonic at a wavelength in therange of about 198-300 nm.
 31. The pulsed laser apparatus of claim 30 ,wherein the nonlinear optical crystal is beta-Ba₂BO₄.
 32. The pulsedlaser apparatus of claim 23 , wherein the laser pulses incident on theat least one optical wavelength conversion means are scanned across thesurface of the at least one optical wavelength conversion means, therebydistributing the thermal energy of the laser pulses over the surface ofthe at least one wavelength conversion means and reducing the powerloading in the at least one wavelength conversion means.
 33. A pulsedlaser apparatus for providing optically smooth ablation of cornea tissuefrom generated laser pulses, wherein each pulse of the laser apparatusprovides an ablation depth of about 0.2 microns or less, and has anablation energy density threshold less than or equal to about 10 mJ/cm².34. The pulsed laser apparatus of claim 33 , wherein the ablation depthof each laser pulse is about 0.05 microns or less.
 35. The pulsed laserapparatus of claim 33 , wherein the apparatus includes a laser emittingpulses having a wavelength in the range of about 198-300 nm and aduration in the range of about 1-5,000 picoseconds.
 36. The pulsed laserapparatus of claim 35 , wherein the lasing medium of the laser isselected from the following group: Ti-doped Al₂O₃, alexandrite, emerald,Cr:LiCaF, Cr:LiSrF, Cr:forsterite, color center lasers, or rare earthion laser media in a solid state crystal host.
 37. The pulsed laserapparatus of claim 33 , wherein the laser emits up to approximately50,000 pulses per second.
 38. The pulsed laser apparatus of claim 33 ,further including: a. means for positioning the laser pulses within aselected area; b. means for controlling the diameter of the laserpulses; c. means for controlling the intensity of the laser pulses. 39.The pulsed laser apparatus of claim 38 , further including means forcontrollably blocking the laser pulses from the cornea tissue.
 40. Thepulsed laser apparatus of claim 33 , wherein the laser pulses have anundeflected position, and further including means for generating avisible guide beam coaxial with the undeflected position of the laserpulses, for aligning the laser pulses on a cornea.
 41. The pulsed laserapparatus of claim 40 , wherein the means for generating a visible guidebeam includes a low-power laser.
 42. The pulsed laser apparatus of claim40 , wherein the means for generating a visible guide beam includes ameans for generating a ring of light adjustable in diameter.
 43. Thepulsed laser apparatus of claim 42 , wherein the means for generating aring of light includes first and second axicon prisms aligned optically.44. The pulsed laser apparatus of claim 33 , wherein the laser pulseshave an undeflected position, further including mneans for biasing thealignment of the undeflected position of the laser pulse with respect tothe cornea by tracking movement of the associated eye.
 45. The pulsedlaser apparatus of claim 44 , wherein the means for biasing thealignment of the undeflected position of the laser pulse with respect tothe cornea includes: a. indicator means, attached to the eye, forproviding visible indications of the movement of the eye; b. sensormeans for detecting the visible indications provided by the indicatormeans and for providing control signals in response to such detection;c. beam positioning means, coupled to the sensor means, for biasing thealignment of the undeflected position of the laser pulses with respectto the cornea in response to the control signals.
 46. The pulsed laserapparatus of claim 45 , wherein the indicator means includes a vacuumeye ring having distinct visible linear eye position indicators.
 47. Thepulsed laser apparatus of claim 46 , wherein the distinct visible lineareye position indicators include: a. at least one first line indicatingposition of the eye ring in an X-direction; b. at least one second line,orthogonal to the at least first line, indicating position of the eyering in a Y-direction.
 48. The pulsed laser apparatus of claim 47 ,wherein the distinct visible linear eye position indicators furtherinclude at least one radial line indicating rotation of the eye ring.49. The pulsed laser apparatus of claim 47 , wherein the sensor meansincludes: a. a first linear array sensor orthogonal to an image of eachfirst line; b. a second linear array sensor orthogonal to an image ofeach second line.
 50. The pulsed laser apparatus of claim 47 , whereinthe sensor means includes: a. a first linear array sensor orthogonal toan image of each first line; b. a second linear array sensor orthogonalto an image of each second c. a third linear array sensor orthogonal toan image of each radial line.
 51. The pulsed laser apparatus of claim 45, wherein the beam positioning means includes at least two orthogonalreflective surfaces positioned by controllable actuators.
 52. The pulsedlaser apparatus of claim 45 , wherein the beam positioning meansincludes an optical means for controllably rotating the image of anincident laser pulse.
 53. The pulsed laser apparatus of claim 33 ,further including means for providing a surface profile of the cornea.54. The pulsed laser apparatus of claim 53 , wherein the means forproviding a surface profile of the cornea generates control signals forthe means for positioning the laser pulses within a selected area, themeans for controlling the diameter of the laser pulses, and the meansfor controlling the intensity of the laser pulses, to limit ablation ofthe cornea to selected areas and to selected depths within the selectedareas.
 55. In a laser apparatus for cornea surgery, wherein the laserbeam has an undeflected position, an eye tracking system for biasing thealignment of the undeflected position of the laser beam with respect tothe cornea, including: a. indicator means, attached to the eye, forproviding visible indications of the movement of the eye; b. sensormeans for detecting the visible indications provided by the indicatormeans and for providing control signals in response to such detection;c. beam positioning means, coupled to the sensor means, for biasing thealignment of the undeflected position of the laser beam with respect tothe cornea in response to the control signals.
 56. The pulsed laserapparatus of claim 55 , wherein the indicator means includes a vacuumeye ring having distinct visible linear eye position indicators.
 57. Thepulsed laser apparatus of claim 56 , wherein the distinct visible lineareye position indicators include: a. at least one first line indicatingposition of the eye ring in an X-direction; b. at least one second line,orthogonal to the at least first line, indicating position of the eyering in a Y-direction.
 58. The pulsed laser apparatus of claim 56 ,wherein the distinct visible linear eye position indicators furtherinclude at least one radial line indicating rotation of the eye ring.59. The pulsed laser apparatus of claim 57 , wherein the sensor meansincludes: a. a first linear array sensor orthogonal to an image of eachfirst line; b. a second linear array sensor orthogonal to an image ofeach second line.
 60. The pulsed laser apparatus of claim 57 , whereinthe sensor means includes: a. a first linear array sensor orthogonal toan image of each first line; b. a second linear array sensor orthogonalto an image of each second line; c. a third linear array sensororthogonal to an image of each radial line.
 61. The pulsed laserapparatus of claim 55 , wherein the beam positioning means includes atleast two orthogonal reflective surfaces positioned by controllableactuators.
 62. The pulsed laser apparatus of claim 55 , wherein the beampositioning means includes an optical means for controllably rotatingthe image of an incident laser pulse.
 63. In a laser apparatus forcornea surgery, wherein the laser pulses have an undeflected position, aguide beam generation system including means for generating a visibleguide beam coaxial with the undeflected position of the laser pulses,for aligning the laser pulses on a cornea.
 64. The pulsed laserapparatus of claim 63 , wherein the means for generating a visible guidebeam includes a low-power laser.
 65. The pulsed laser apparatus of claim64 , wherein the means for generating a visible guide beam includes ameans for generating a ring of light adjustable in diameter.
 66. Thepulsed laser apparatus of claim 65 , wherein the means for generating aring of light includes first and second axicon prisms aligned optically.67. A wavelength converter for laser pulses including: a. laser meansfor generating laser pulses which deflectable around a pivot point froman undeflected position; b. at least one optical wavelength conversionmeans, for converting the fundamental wavelengths of original incidentlaser pulses into corresponding second harmonic wavelengths; c. a firstlens, positioned at about the distance of its focal length from thepivot point of the laser pulses, for receiving incident laser pulses andorienting the incident angle of such-laser pulses relative to a firstsurface of the at least one optical wavelength conversion means suchthat the incident angle remains constant regardless of the angle ofdeflection of the laser pulses; d. a second lens, positioned at aboutthe distance of its focal length from the at least one opticalwavelength conversion means, for receiving incident laser pulses fromthe at least one optical wavelength conversion means and for orientingsuch laser pulses such that the laser pulses are re-collimated afterpassing through the second lens; e. wavelength separation means forspatially separating the fundamental wavelength of the original incidentlaser pulses from wavelength converted laser pulses exiting from thesecond lens.
 68. The pulsed laser apparatus of claim 67 , wherein asecond optical wavelength conversion means is adjacent to a firstoptical wavelength conversion means.
 69. The pulsed laser apparatus ofclaim 67 , wherein the first lens and second lens are the same lens, andfurther including: a. reflecting means, adjacent the wavelengthconversion means, for reflecting laser pulses exiting the wavelengthconversion means back through the wavelength conversion means to thelens; and b. angled reflecting means for reflecting wavelength convertedlaser pulses exiting the lens.
 70. The pulsed laser apparatus of claim69 , wherein a second optical wavelength conversion means is adjacent toa first optical wavelength conversion means.
 71. The pulsed laserapparatus of claim 67 , wherein the optical wavelength conversion meansincludes a nonlinear optical crystal.
 72. The pulsed laser apparatus ofclaim 71 , wherein the nonlinear optical crystal is beta-Ba₂BO₄.
 73. Thepulsed laser apparatus of claim 67 , wherein the laser pulses incidenton the at least one optical wavelength conversion means are scannedacross the surface of the at least one optical wavelength conversionmeans, thereby distributing the thermal energy of the laser pulses overthe surface of the at least one wavelength conversion means and reducingthe power loading in the at least one wavelength conversion means. 74.The pulsed laser apparatus of claim 1 , further including: a. means forpositioning each of the laser pulses with respect to a surface of amaterial; b. means for ablating the surface of the material to anoptically-smooth finish by depositing on the surface a plurality oflaser pulses in at least one layer, the laser pulses in each layer beingpositioned in a regular pattern and generating an etch profile in thematerial.
 75. The pulsed laser apparatus of claim 74 , wherein theablation depth of each laser pulse is about 0.05 microns or less. 76.The pulsed laser apparatus of claim 74 , wherein the apparatus includesa laser emitting pulses having a wavelength in the range of about198-300 nm and a duration in the range of about 1-5,000 picoseconds. 77.The pulsed laser apparatus of claim 76 , wherein the lasing medium ofthe laser is selected from the following group: Ti-doped Al₂O₃,alexandrite, emerald, Cr:LiCaF, Cr:LiSrF, Cr:forsterite, color centerlasers, or rare earth ion laser media in a solid state crystal host. 78.The pulsed laser apparatus of claim 74 , wherein the laser emits up toapproximately 50,000 pulses per second.
 79. The pulsed laser apparatusof claim 74 , further including means for controlling the diameter ofthe laser pulses.
 80. The pulsed laser apparatus of claim 74 , furtherincluding means for controlling the intensity of the laser pulses. 81.The pulsed laser apparatus of claim 74 , further including means forcontrollably blocking the laser pulses from the material.
 82. The pulsedlaser apparatus of claim 74 , wherein the material to be ablated is acornea, and further including means for providing a surface profile ofthe cornea.
 83. The pulsed laser apparatus of claim 82 , furtherincluding means for determining the coordinates of cornea tissue to beremoved from the surface of the cornea.
 84. The pulsed laser apparatusof claim 83 , further including: a. means for positioning the laserpulses within a selected area of the cornea; b. means for controllingthe diameter of the laser pulses; c. means for controlling the intensityof the laser pulses; wherein the means for providing a surface profileof the cornea generates control signals for the means for determiningcoordinates, and wherein the means for determining coordinates generatescontrol signals for the means for positioning, the means for controllingdiameter, and the means for controlling intensity of the laser pulses,to limit ablation of the cornea to selected areas and to selected depthswithin the selected areas to obtain an optically-smooth finish for thecornea.
 85. The pulsed laser apparatus of claim 74 , wherein the etchprofiles have a Gaussian intensity distribution.
 86. The pulsed laserapparatus of claim 74 , wherein the etch profiles have a super-Gaussianintensity distribution.
 87. The pulsed laser apparatus of claim 74 ,wherein the regular pattern of each layer is predetermined.
 88. Thepulsed laser apparatus of claim 87 , wherein each layer has an originand the origin of each layer after the first layer is offset from theorigin of the immediately subjacent layer so as to maximize smoothing ofthe surface of the material.
 89. The pulsed laser apparatus of claim 88, wherein each layer after the first layer has an origin at anequivalent offset point defined relative to the origin of the firstlayer.
 90. The pulsed laser apparatus of claim 89 , wherein the etchprofiles having an approximately circular cross-section with a radius r,each layer comprising a substantially non-overlapping, substantiallycontiguous hexagonally-packed array of etch profiles, the center of eachetch profile in a layer being spaced approximately a distance D, equalto 2r, from the center of each other etch profile in the layer, whereinthe origin of each layer is offset from the origins of subjacent layersand positioned at an equivalent offset point so as to minimize themaximum crest-to-trough distance in the tissue produced by the pluralityof layers of etch profiles.
 91. The pulsed laser apparatus of claim 90 ,wherein the center of each etch profile in a layer is spacedapproximately a distance D, equal to 2r, from the center of each otheretch profile in the layer, and wherein: a. a first layer has an originA; b. a second layer has an origin B1 located at a point equivalent to apoint approximately midway along a line connecting origin A to thecenter of a first adjacent etch profile of the first layer; c. a thirdlayer has an origin B2 located at a point equivalent to a pointapproximately midway along a line connecting origin A to the center of asecond adjacent etch profile of the first layer, the second adjacentetch profile being adjacent to the first adjacent etch profile; d. afourth layer has an origin B3 located at a point equivalent to a pointapproximately midway along a line connecting the center of the firstadjacent etch profile and the center of the second adjacent etchprofile.
 92. The pulsed laser apparatus of claim 87 , wherein the etchprofiles have an approximately circular cross-section with a radius r,and further including: a. means for producing a first level pattern ofetch profiles on the surface of the material, the first level patterncomprising one etch layer having an origin A and comprising asubstantially non-overlapping, substantially contiguoushexagonally-packed array of etch profiles, the center of each etchprofile being spaced approximately a distance D, equal to 2r, from thecenter of each other etch profile in the etch layer; b. means forproducing a second level pattern of etch profiles on the surface of thematerial, the second level pattern comprising three etch layers, eachetch layer comprising a substantially non-overlapping, substantiallycontiguous hexagonally-packed array of etch profiles, the center of eachetch profile being spaced approximately a distance D, equal to 2r, fromthe center of each other etch profile in the etch layer, wherein: (1) afirst etch layer of the second level pattern has an origin B1, theorigin B1 being located approximately midway along a line connecting theorigin A of the etch layer of the first level pattern to the center of afirst adjacent etch profile of the etch layer of the first levelpattern; (2) a second etch layer of the second level pattern has anorigin B2, the origin B2 being located approximately midway along a lineconnecting the origin A of the etch layer of the first level pattern tothe center of a second adjacent etch profile of the etch layer of thefirst level pattern, the second adjacent etch profile being adjacent tothe first adjacent etch profile; (3) a third etch layer of the secondlevel pattern has an origin B3, the origin B3 being locatedapproximately midway along a line connecting the center of the firstadjacent etch profile and the center of the second adjacent etchprofile.